After Richard III's son died, some claim that Richard made him his provisional heir, though there are source issues. He was also,in one account, said to be 'unable to distinguish duck from goose' or some such. Some read that as his being mentally wanting, others think it's merely the natural state of someone kept imprisoned as long as he was by Henry VII.
Dickardians love him though, pointing out that post Titulus Regis Richard's motivation to kill the Princes in the Tower applied equally to his other nephew, and the fact that Richard apparently treated him very well and obviously did not kill him could be said to speak volumes.