Releasing a model is fine, only if it is separate from the game. The only time you would potentially run into issues with publicizing a character model from an IP is if it were ripped straight from a game. He modeled this himself, so if he chooses he can put the character up for download. Where it starts to get into infringement territory is if he were to release this character in addition to the gameplay mechanics using the Attack on Titan name. And keep in mind this is also dependent upon the company who owns the IP. Some companies are tolerable of fan games, but others who are invested in the game market themselves might not be so open to the idea. Ultimately as said he’s going to want to talk to the owners of the IP in order to determine whether or not he could release a fan game.