One of the things you can do for the fluid build up is to keep his protein intake up. The protein will help draw the fluid back into his blood stream. It's not an instant fix but will help with the process. Meats, eggs, cheeses, cottage cheese, yogurt.... all that good stuff. If you are vegetarian then rice and beans, tofu and other protein substitutes. Protein shakes are great too. There are tons of ways to make them but we used frozen fruit, ice cream, yogurt and milk. Also we added whey protein powder....my husband liked the milk and cookies flavor and then I hid plain, unflavored in things like mashed potatoes, gravy and soups.
As for the tummy problems, I would ask the doctor. There are lots of things that could cause that. If he has had recent antibiotics that can cause issues like that. Probiotic might help regulate things (ask at the pharmacy it should be over the counter). If it continues have the doctor check a stool sample and they can tell a lot from that.
Hope this helps.
Any advice given is based on my experiences and should not be substituted for any medical recommendations. Please speak with your provider before making any changes.