What Causes Red Blotches Along With Swollen Lips And Face?

After eating a cheese quiche my son came up with red blothes and his lips and face was swollen, was breathing fine though. Other things give him red blothes e.g ketchup, lasagne and some fruits. Never has a solid stool. Always soft, foul smelling and sort of greeny/slimey. What could this be? Should I be cutting things from diet? Allergy testing?