Well, unless you are in a monogamous relationship with someone, I'm not sure you can ever fully trust someone's word. BUT, the good news is, only the head of the penis needs to be covered. If it slips up, you're still protected.
In theory, if he really is on Prep, then he would be HIV free. And also, one time sex is a low risk exposure. HIV is very fragile virus.
So, it's your judgment call about the integrity of the condom and whether you believe him or not regarding Prep.