Absolutely no risk of anything.
Buttocks (clothed or unclothed) do not transmit HIV.
He could have sat on your face and you would have not been at risk.
Thank you for all your replies. I feel much relieved now. I have one last question. If it had been the front, that is the crotch region would it have been an absolute no risk situation as well, even if the pants might be unzipped? I live in very crowded places and situations like these present itself from time to time.
HIV is transmitted from:
Unprotected sex (vaginal and/or anal)
Mother to unborn/nursing child
Sharing IV drug needles