HIV status is confidential medical information. No health care provider can give that information to your employer.
HIV status would have gotten you barred from entering the US until very recently. It is no longer a bar to travel to the US so it can't be used against you for a green card, etc. However, you have a long ways to go before you need to worry about a green card.
You need to get the job offer and a visa to work in the US in the first place.

