Was that the only thing that got on his mouth (the hummus?). If so, yes, i would cut it out. And YES, I would for sure find a ped allergist to get tested.
I know there are people/dr's who don't think kids get allergies - that is so crazy! My own ped says that about environmental allergies, but not food allergies.
Can you get an appointment? He is getting some type of contact reaction, and yeah, you don't want to keep pulling things out of his diet. I think taking out the hummus/citrus for now is a great idea. Could just be an irritation as citrus is acidic. That happened to my DD1 w/mango. She's fine with it, just would eat so messily and irritated her skin..