Been there.....I feel your pain!
I would definitely not get him neutered until he is completely stabilized. Surgery will pull down his immune system and could possibly make things worse.
Keep high fats away from your baby. Check all packages or go to the sites and look for specifics. Have you tried a good quality digestive enzyme/probiotic yet? That and homeopathic remedies literally saved my dogs life!
Here is a good DE/PB: Sunday Sundae by Carrmspac here on the blog: Sunday Sundae Nutritional Supplement