A key question is whether your grandson and his mother are comfortable disclosing his disability at college. It’s a long road from diagnosis to acceptance.
If he is comfortable with disclosing, he might want to check out the local Community College (or University of his choice) and talk to someone in the Disability Services Department. They can make sure he has the accommodations in college that he needs to succeed.
He also might find a support group for young people with ADHD in his area. Meetup.com is a good place to look. Talking to other people with invisible disabilities about disclosure can help him decide if it’s right for him.
Just saw an interesting article on the subject. http://oregonstate.edu/ua/ncs/archives/2017/aug/self-identifying-disabled-and-developing-pride-disability-aid-overall-well-being
It is not an easy decision to make. Help him find any resources he can to not make the decision alone. All the best.