My son was diagnosed with colitis. Is that something I should worry about?

Yes. I assume you are talking about ulcerative colitis(uc). Uc is chronic condition and persist life long. It gets better with treatment. There are very good treatments. However needs maintainece treament to keep it in remission. If not treated properly it can have serious complications.
Depends. Depends on what type of colitis. There are several forms each of which has concerns for his future health. Please be more specific in the type of colitis.