This question should have been on the HIV forum; sorry I didn't think of this before responding. Because it's the wrong forum, there won't be any follow-up discussion.
Welcome to the HIV forum.
Certainly you are correct on one point: This is a unique question, indeed the first time I have ever been asked -- on this fourm or anywhere else -- about GERD as an HIV risk factor.
MIght it actually be a risk factor? I doubt it, but to my knowledge there has been no research on it. But the worst case scenario might be a doubling of the risk, if exposed to the virus. (That's the risk with other inflammatory conditions, e.g. herpes, and in men who are uncircumcised.) But I stress "if exposed". If your partner isn't a gay/bi man, injection drug user, etc then most likley he isn't infected. And in any case, oral exposure to infected semen appears to carry something like a 1 in 10,000 risk of infection. If you double that, it's still only 1 in 5,000. That's equvalent to giving oral sex to HIV infected men once daily for 14 years and maybe never becoming infected.
If you have strong reason to believe your partner had HIV, then having an HIV test would make sense. But that advice would be no different with or without GERD. If he is at low risk, and especially if he says he doesn't have HIV (did you ask?), I wouldn't worry about it and would not recommend testing.
I hope this helps. Best wishes-- HHH, MD