This guy is an idiot. There is absolutely no reason for him to be unable to study sorcery if he revealed himself as a reincarnator. If anything it would make it easier as people from all over the world would be flocking to him to see how their theories checked out at bare minimum while also having people with ancient/forbidden texts trying to curry favor by bringing them to him.
I myself am not immune to derpy moments, but this is so stupid that I question the character's supposed intelligence and/or suspect some sort of brain problem. The worst case scenario here for him is he has to build magic back up to the old levels in order to make a cadre of magical researchers and that is still more effective than the equivalent of watching a bunch of kindergarteners trying to explain math to each other.