I know it's super frustrating and of course your worried about your kiddo, but there isn't really anything they can test for :( like other posters, try to think of anything new your LO may have been exposed to, on their body or on yours, any new foods if you've started solids early, anything out of the ordinary. But even then, allergies are so crazy and sometimes unpredictable so they can even develop them at any time to things that you use every day. Hope the antihistamines helped and its an isolated incident! Good luck!

Did you put any new clothes on your LO without washing them 1st? I made that mistake with my 1st child and ended up in the hospital cuz he was covered in hives. Or have you changed any soaps softeners ect? 

My sister looked like a tomato she was so covered with hives, they didn't know what caused it. They can be caused by a million different things, it's hard to determine without exposing him to every little thing he was recently exposed to. Glad he got to go home and hope he's feeling better soon!

When did your LO have vaccinations last? If it was recent that could definitely be the cause.