Hi, just to clarify - your DS is having no milk, soy or wheat at the moment? What makes you think you would have a problem to wheat? Is he still vomiting a lot? Have you tried an elemental formula such as Neocate?
As with any trial you would look at out for signs of an intolerance or allergy - hives, vomiting in a fast reacting allergy, stomach pains, gas, eczema etc in an intolerance. It may well take a few days to show itself so worth not introducing anything else new for a few days after as it can then get confusing.
Does that help?