M.M.
When I was 23 I was stuck in between that proverbial rock and hard place. I just needed mom and dad to make the decision for me. It was too hard for me even though I was considered an adult.
Give him the advice. Tell him to live his life and go to college and be whatever he wants to be.
If he doesn't then his grandma will not be happy with him, she would want him to be happy and fullfill his life ambitions, even if she dies while he is in the process.