It really depends on where you live. There are 4 states that allow an adotpee access to their original birth certificate (sadly, there are 46 states that don't).
If he can't get that, the next step would be to contact the agency that facilitated the adoption, and ask for his non-identifying information. There might be information in there that can help with his medical issues.
Maybe there's something in his file, as well; she could have left a letter or something for him.
Some agencies provide search services, in MN where my adoption was handled, the agency charged me a fee to locate my mother and contact her for me (Warning: using anyone as an intermediary is bad business, you just never can trust the motives of the intermediary.. ESPECIALLY if it is the agency itself). But if you have no other options, you have to do what you have to do.
If it is a medical emergency, you might be able to petition a judge to open his records, but that is very hard to do unless you luck out and get a sympathetic judge. Remember, adoption is big money for the legal system, so they are not very sypathetic when it comes to opening records.
Keep trying at his mom. Try to find his adoption papers in the house. Talk to other, older relatives and family friends who were around during the time of his adoption. Someone might know something...you never know.
And REGISTER...on adoption reunion registries, see if your state has a registry, put his name and stats OUT THERE in case she is looking for him!!!
Good luck, and I suggest contacting Gershom (she answered above), I know her well and I know that she is a SUPER source for adoption search information. She knows a lot more than I do!
I wish you and your boyfriend the best of luck!