There could be any number of reasons why your friend doesn't get fully erect. My first question would be whether he is able to get fully erect when he masturbates. If so, it may not be a physical problem as much as a mental one--performance anxiety, for example.

If this is a new hook-up situation, he may not feel that comfortable with you yet. And there is always the possibility that he is just not that interested in girls.

If he is masturbating a lot when he's alone, that could make him less likely to be able to achieve a full erection when he's with you. If you think this is the case, ask him to try holding off on masturbating for half a day before he sees you and see if that helps.

There are also physical problems, such as diabetes, that can make it difficult to become erect.

I'm not sure how much effort you want to put into getting to the root of the problem, if you're not in a relationship with this guy, but if the relationship progresses and he continues to be semi-hard during sex, suggest that he bring it up the next time he gets a health check-up.

Take care.