If your child is not receiving accommodations from his college yet, apply for them through the college’s Office of Student Disabilities (or similarly named office).
If your son is approved for accommodations, request that he receive extended time on tests, testing in a separate and quiet location, having a note-taker in class, priority class registration, and having a reduced course load count as full-time. In my book Making the Grade with ADD, there is an entire chapter devoted to college accommodations.
Also consider hiring an ADHD coach to help your son create a structured schedule (people with ADHD thrive with structure), learn effective studying techniques, and be accountable to an impartial third party for completing assignments and projects. In addition, an ADHD coach can help your son break large projects into smaller, more manageable pieces.