What is he eating? Try not to limit food intake, even to avoid vomiting. He won't regain strength if he does not eat properly, especially now when the body needs nourishment. Three miles is a lot of walking. Protein, at every meal, is vital for the damaged and wounded tissues to begin healing, and to regain his strength and health. I know hospital food is not the best. For breakfast I used to see a dry bagel with a hard boiled egg on the plate. That was all. Lunch and dinner were worse. Who can get strong on that? I lost 15 pounds in one week. The food has to be appetizing.
Even if you have to bring it from home, see if he can eat a good hot meal, or snacks. Make sure there is a protein at Every Meal: fish, chicken, beef, turn some canned tuna or salmon into tuna salad on toast, or egg salad that is soft and easy to digest (make it with half mayo, and half Greek yogurt for some protein), peanut butter on toast with sliced banana. Will he drink a protein smoothie, such as Greek yogurt with banana (or an avocado) and milk? Will he eat hot soup? Hot oatmeal (make it thin, not thick) with peanut butter or honey swirled in for flavour.
Most hospitals have arrangements for a special home care / community nurse to visit you at home to check the wound, change the appliance, address your concerns. Check if they have home care process before you are discharged.
Try to take one day at a time. Don't let stress, worry or anxiety overtake you at any point. That will just make you or your son freeze up and you might not ask the right questions or think clearly. Make a list of what you need to do or ask that day, and do your best. Don't stress because guts do not like stress and tie itself in knots. You know the feeling. You will have follow up appointments and they will do bloodwork. If they forget to give you an appointment date when you're discharge, ask for one. My first appointment with my surgeon was one month after discharge.
Drink warm broth, salty, because he will need salt. You can monitor your own (de)hydration. If his urine is dark coloured, he's not getting enough fluids and is a sign of dehydration. Canned broths, until you have time to make your own, will provide lots of sodium and fluid. Throw in some chicken or rice or potato in the broth. Plain water will just go right through him, try canned broths.