Athos was not a sensor, nor did he have anything but his golden hues, but he was a warrior trained in more ways of combat than most, something his son knew. And as the genjutsu was cast, Athos felt something off, yes... But he had let his guard down.
Was this truly happening? His firstborn son was before him, able to move forward together like they had many years before... But this... Athos had been trapped in the genjutsu, there was no denying that, but as Ryoji moved to take the mask off his face to reveal a smile... Well, perhaps this was a twisted thing for a father to say, but it immediately began to rub him the wrong way. No, that wasn't right. This wasn't right. None of this was how things would go, not after all these years, where was the battle? The dramatic resolve? The betrayal?!
It was true that Athos no longer had the help of a Bijuu to help him break out of the genjutsu, and even the golden eyes of the Raikon Clan didn't allow him to actually see chakra, but he didn't need any of that. Athos had gone against Bocchiere, who had broken his mind through Tsukuyomi, and through the literally torture of his children in front of him. Athos Raikon had watched as his first child slaughtered his siblings endlessly, for no other reason than spite and hatred. He has died and tortured by the Shinigami relentlessly, only to be brought back time and time again. This didn't give him some sort of immunity from genjutsu, but it gave him indomitable willpower. Willpower that has been responsible for saving Ryoji's life, despite whatever befell Athos before they split apart. This was too perfect, from the moment his child took off his mask. He was no fool, not when it came to Shin Raikon, his son.
But even as he came to the realization that he had been duped, Ryoji just about to begin his speech, he felt chakra surge into his body. You see, Athos had extended his left hand for several reasons, all of them being connected to the tattoo on his arm, though beneath the ink were long and complicated formulas for fuinjutsu intertwined together. The chakra chains seal was indeed a seal capable of binding Bijuu, but it was a seal that required very little chakra to actually be used. The Fuinjutsu on Athos' arm allowed him to absorb chakra directly from a person, or in this case, from the chakra that the seal was going to be/is made of. And with this, Athos would also be broken from the Genjutsu, as Ryoji's chakra would be absorbed into his form and disrupt his chakra flow, though not in a negative way, it was still a disruption. So even though Athos had/would have come to the realization himself that he was duped, Ryoji would have unknowingly broken Athos out of the Genjutsu himself by placing the seal/touching him.
But there was another reason that Athos had chosen his left arm to extend towards his son, another one that stemmed from that same Fuinjutsu on his arm. See, Athos had planned to be the one doing the duping this time around but like father, like son, it seems. So as the Genjutsu was broken, Athos had simply planned to use his previous plan, seeing as it was the quickest route. Ryoji had already willing been touching Athos, so the next parts of his plan would be easy. Athos' left hand had been right next to Ryoji's own left wrist, the two being less than inches apart(tested it out IRL to make sure), so using that he moved to grab his wrist as well, an action that would happen so fast, considering Athos' master of fighting styles like Iado and other quick striking methods, it would take maybe the tenth of a second, if even that, considering that the two of them were already touching one another. Even with the Sharigan, Ryoji had made a point of trying to lean forward and peer into Athos' face, meaning that he wouldn't be able to pick up on the faint gesture by Athos until it was too late.
Usually touching him like that would be meaningless without some type of handsign or flare of chakra but on the palm of Athos' hand, attached to the rest of the Fuinjutsu that was on his arm, were three previously marked down seals that formed an Uzumaki Triangle Sealing technique, which was the more advanced and efficient version of the Uzumaki Sealing Technique. If one of these seals can instantly seal away one person by touching them with it, see a specific previous fight, then the advanced version that is comprised of three seals and meant for sealing much larger areas with much larger success would have no problem absorbing a man just as fast, if not faster(if that's even possible), than the generic Uzumaki Sealing Technique.
But if the fact that Ryoji and Athos were already touching by Ryoji's consent for this to be effective wasn't enough, Athos hadn't stopped there. Seeing as they were, again, already touching, he would use that to connect their shadows. Now, Athos wouldn't actually control Ryoji's shadow but attach them so that the moment he tried to physically move away from the attack he would find himself unable to do so, though one could wonder how he would even notice any of this seeing as it happened all in a single moment, if that. Athos made sure not to physically affect Ryoji's motions with his shadow as they connected but even if Athos did move Ryoji, again, it was such a small and faint movement it would be noticed much too late to do anything for his son.
That being said, if Ryoji, with his eyes directly trying to meet Athos', did manage to see the extremely faint and swift gesture by Athos to grab his wrist and tried to escape via, let's say, Kamui, it would be unsuccessful as well, since the two of them would still be touching and it would do nothing to stop Athos from doing so. And even the long range version of Kamui wouldn't be activated fast enough before Athos could grab Ryoji and seal him away.
All of this happened in a single moment, seeing as Ryoji would be unaware of the newest upgrades done to Athos' Fuinjutsu to take it into its 'Perfect' form, and the Genjutsu being broken without the knowledge of the man who cast it, seeing there would be no physical or obvious indicators that Athos would have been broken out of it. But even if for some god awful reason Ryoji managed to survive this attempt to be sealed, Athos made sure to activate his Sharingan only after he tried to seal Ryoji away, just as the man had begun trying to push Athos further into a genjutsu, an attempt that would fail seeing as he had broken out of the one he was trying to further succumb him to. And with how fast Athos' motion worked, even if Athos were to somehow fall victim to the Sharingan, it would be after Ryoji was sealed, if that was even possible.
[Uzumaki Triangle Seal]
[Shadow Possession, same time as above]
[Activate Sharingan, less than a moment after the hopeful sealing, as to not give away that he was broken out of the Genjutsu]