What's causing the hives?

Recently we've noticed that Nate has been coming out in hives soon after eating. We have begun to try and work out what is causing these hives. Nate already avoids dairy and largely soya due to intolerances to them I'm not looking forward to adding anything else to that list.

Last years hives

We have on our list of potential suspects tomato, mayonnaise or brown bread. We're not 100% sure that he has had the same food each time the hives have come out. He loves tomatoes but noticed this time last year every time he had them he came out in hives so we avoided them until early this year when there was no reaction. 

This mornings hives

We are thinking it may be brown bread as we recently have changed to brown bread due to the recommendation of the nutritionalist and this morning while having his toast we noticed his legs were covered in hives. However, he hadn't eaten any brown bread when he came out in hives yesterday so we may be looking for something else as well.