If you’re a Top, try holding his legs back (or putting them over your shoulders) to control how far down you go. Keep licking, kissing and sucking down toward his balls, then move on to his perineum and even further to his asshole. If he’s keen, moan and wriggle your asshole up a bit so you can get deeper into his anal.
Of course, you’ll need to take care of anal hygiene — condoms and lots of lube are in order. Also, it goes without saying that if you’re HIV-positive, anal isn’t for you unless you’re taking daily antiretroviral therapy (ART).