I would say that he is only under obligation to live a Catholic life in relationship with Christ insofar as he can. That means he confesses whatever sins he can remember. Even for those of us who do NOT get stricken with such severe memory loss, if we do not remember some sin or other, then it is covered when we confess the sins we DO remember. That’s why, after a good confession, we are encouraged to say we are sorry “For these and any sins I cannot now remember.” God will not condemn a soul to hell for not confessing a sin that he couldn’t remember. And since a person can confess venial sins in confession if he wishes, I am sure that a person in those shoes could soon find something, however small, to bring to confession if only for the sake of being able to gain the assurance of knowing that ALL his unremembered sins would be forgiven in the sacrament as well.
Now, if someone from the man’s past informed him of the sins he had committed, and he had good reason to believe this person was telling the truth, it might not hurt if he would bring up those things in confession while informing the priest that he was confessing these things “just in case” he had never confessed them before. Perhaps this would be the confessional equivalent of a “conditional baptism.”
The important thing is that, with everything he DOES know, he live the fullest Christian life. If he is doing that, then God will not allow his soul to be lost for things that are completely beyond his control.
Blessings in Christ,