Hello Nancy
I'm no expert. I'm intolerant rather than allergic myself, but did not become markedly so until adulthood.
1) Could Tyler genuinely have a bug at present? Have you ruled out e-coli etc?
2) Being intolerant is better than being allergic because the reactions are less likely to be life threatening. The trouble is that the medical profession isn't as good at treating intolerances as allergies.
3) You switched Tyler to soy and he was fine for a while and then started exhibiting symptoms. Maybe he has a low tolerance for a lot of things and you've now reached his limit on soy and you need to give him a rest from soy. There are also milk substitutes around which are based on rice and other grains. I've no idea if they're suitable for young children, but if you can find several products which he can tolerate, maybe you can give them in rotation so that his tummy doesn't get enough of any one to react to it. Soy is one of the common foods that people are intolerant of
If you want to try 3) you should take advice from a qualified person to ensure that Tyler is getting adequate nutrition.
It sounds like you're intending to take advice from a peds gastroenterologist in the near future. Maybe he/she can offer some advice, or maybe you're going to need to consult a nutritionist as well.
Helen