Try finding a warm pool. Easter Seals, YMCA, etc. Start with trying to walk in the shallow end, gentle stretches, etc.

At home, instead of trying to push yourself past point that you flare, walk a short distance, rest, repeat.

Your child is an adult. (I have a son in college as well.) Sacrificing an opportunity for you to regain your health isn't good for anybody. There are always ways to make things work financially while in college--hunting down scholarships to apply for, student loans, taking a semester off for an internship, etc.