That is a hard situation. My DS is only mildly allergic to corn, cow/goat milk products, and tomatoes. Soy used to bother him also, but I still haven't reintroduced it. I know it is really difficult, but please keep breastfeeding him. My boy wouldn't take the nutramigen, plus I just wasn't sure that would be better. There is a yahoo group called "foodlab" that was a great help to me. I went there when I first started eliminating foods to help my son. Just a warning though, when you post there, don't bemoan the loss of any favorite foods because most of the people on the list have been there, done that. In other words, go looking for solutions, not sympathy. Sounds harsh, but it is actually really a great list with lots of practical support.
I think that once you eliminate corn and have been off the other foods for at least 4-6 weeks, your son will gain weight better. He will probably want to nurse more once your milk doesn't bother him so much. Poor little guy... reminds me of when I would eat a tamale then wonder why he would cry at the breast and projectile-spit up. It is hard to have that many restrictions, but try to look at this as an opportunity to explore foods that you never thought of while helping your son.
This is an aside, but last week I went on a business trip for the first time (DS is 21 months). He is still nursing, so I took my pump to keep my supply going. Knowing that I was going to pump and dump, I ate some corn chips, had a cheeseburger, and some ice cream. I have to say, the anticipation was much greater than the actual experience. I was pretty dissappointed that none of those things were as good as I remembered.
Good luck!