Oh, good -- not that he's allergic, but that it *was* obvious ingestion and probably not contact.
The PP who is also allergic to finfish is in an unfortunate minority; true finfish allergies are pretty rare. An ingestion allergy to shellfish usually isn't too awful; as long as you are careful about cross-contamination at places like seafood markets it shouldn't be really difficult to avoid ingesting shellfish products.
Remember, no black and white pasta if you're allergic to molluscs. (I know someone who got tripped up by that one; she didn't realize the pasta sauce contained squid ink.)
Thanks! I'll file the 'black and white' pasta info and tell my son.