My 15 month old is being referred to an allergist?

my doc says I need to tell them what type of allergies I suspect because they can't test for every single allergy. He has stomach issues (restless sleeping, gas etc) and his bowel movements are hard and sometimes blood tinged. I am mainly concerned with food allergist and my biggest suspects are cow's milk, wheat, oats etc. Can you think of any other popular food allergies I should consider (he's not allergic to peanuts)? Thanks