I'm afraid it does sound like some sort of allergic reaction.
Things that people are most likely to react to are: seeds, nuts, peanuts, sesame, soya, cows milk, eggs, fish, shellfish, citrus fruits, tomatoes.
You can react to anything though and you would already know if he had allergies to most of those (although new ones can develop). Maybe a seed/ different type of nut?
Most people react on the second time of eating something (not the first- usually fine)
Does he have any history of allergies including asthma or eczema or hayfever. Other members of the family having any of these also predisposes someone to have an allergy.
Did your ds have any other symptoms of allergy- a rash, hives, shortness of breath?
I would make an appointment with your GP as nottirednow suggested. But be prepared to fight for a referal to an allergy specialist.
Hope that helps and I hopes its something else altogether.
A couple of years ago my dh and I both had a terrible tongue problem - like geographic tongue with no other major symptoms so it could well be a virus or something else especially if he hasn't been atopic before.