My man is quickly coming up on 50, and his sex drive is still plenty strong. In fact, he says we have a better sex life together than he has had in past relationships. Being 38 years old is not "older" in my book, either.
First question: did your bf have a higher sex drive before, and this is a change you've seen during your time together? Or did your bf have a low sex drive when you met him? He may just have a lower sex drive than you normally. Doesn't mean anything is wrong with you or with him, just that you might not be a good match in that department.
Other than that, if he used to have a high sex drive, and this is a change you've noticed - it's time for a check up! Lots of things can cause a change in sex drive, from depression to low testosterone. Any changes in medication can affect it too. So if this is different from how he used to be, it would be a good time for him to get that check up he's probably been putting off!