Erectile dysfunction

Ok, so I know this isn't a rare issue, but my boyfriend is only 20 and it's happened several times that we're in the middle of getting it on and he goes soft. He slept around quite a bit before me and neither of us is used to having sex with someone we actually care about (yay for becoming more of a functional human!) so I'm wondering if he just feels pressured because of that. How do I talk to him about this? I don't want him to feel bad, but I also want to know if it's psychological or physiological