HOLY ****!!!!!!! I cant answer a sing question tonight without someone repeating what Im saying... f this
Has he been eating healthy? Has he been drinking a lot of fluids with caffeine>? If so have him limit it. And have him do like 15 or so squats before intercourse. Also is it a possibility he also masturbates before you do have sex? If so that itself can be a key contributor. Let me know how it goes
There could be something medical going on, but there is a possibility that he's spilling semen somewhere else, like masturbation. I have been in situations where I have masturbated, then later having sex. I didn't have a lot of semen when complete. I've never been asked, but I'm sure my partner was wondering what was up, just like you.
Does he have a high sex drive? How often are the 2 of you active?