I'm a gay guy and i had my first sexual encounter the other day. I sucked his dick for the majority of it but at one point he convinced me to shove just the tip of his cock inside of me, no condom. He didn't ejaculate and we only did that for like a minute before returning to oral sex. How possible is it that i may have contracted HIV? I literally don't know the guy and although he says he doesn't have it, i'm still worried.