I'm sorry Harley has been sick.
Believe me, I know how frustrating it is to deal with stomach issues and an active dog like Harley and Jackson are. Jackson had an incident in late July with a bad stomach possible pancreatitis episode and I spent $1600 on his medical bills (also a student like you!) I kept him pretty much exercise-free for about 2 full weeks to give his whole system a rest. Then we slowly started our walks again and by the 4th week he was totally back to normal. But I think I agree with fasting him for 24hrs and just letting his tummy get settled and see how that goes. The hardest part for me was just keeping him settled - we went for a few car rides now and then just to get him out of the house. But I really think all the rest as well as just eating kibble (no treats, etc) helped.