You are an excellent mom, littlebitsmommy, do not ever forget that. When I was sick as a kid and on prednisone I had all of the issues your son is having now. As difficult as it was to go through it myself I know in many ways it was more difficult for my parents to deal with because they saw me going through everything and wanted to "fix it". Not being able to fix it for months and years was so hard for them, I know. As awful as the side effects of prednisone are it really does work. It is a very old drug but still used today.... there is a reason for that.
I am so sorry your son is in pain. I know he cannot take any opiates because of his GI issues...... I have chronic pain that was helped by Nortriptyline (25mg dose- very low). Although I did not have it available to me when I was on prednisone as a child, I have taken it with prednisone as an adult and it did pretty well. It has been studied for use in children (mainly for ADHD, bed-wetting, etc) so maybe your son might benefit? For children the dose is around 5-10mg.
How long has your son been on the prednisone? Some of the side effects get a bit better over time (i.e. ability to sleep).
I agree wholeheartedly about the post on low salt snacks. High salt snacking just makes the water retention soooo much worse. I ate a lot of Jello Jigglers. They are fun fingerfood and if he can handle the sugar-free version are nearly calorie free so he can eat a lot of them. Honestly though, for some reason, I craved meat. Specifically beef.
You will get through this. Know this is not going to last forever. You are doing the perfect things for your son right now. Your support of him and getting him the best health care show your strength as a person and as his Mom.
Wishing you and your entire family all the best!