Two of a Kind (1982)

Nolie has just turned 21 but is retarded and acts more like 8. While claiming that she only wants to help him, his "smother" actually likes things that way. One day Nolie visits his beloved grandfather in a rest home and is horrified at how the old man has deteriorated, vegetating in his wheelchair. He realizes that this is partly due to the medication the nurses are giving him. Nolie decides that he must rehabilitate Grandpa himself, since no one else cares. Furthermore, he and Grandpa must keep their project a secret. It's a job for a man, and time for Nolie to become one.

Unfortunately the movie Two of a Kind is not yet available on HBO Max.