I don't know how he wouldn't know unless he has some kind of loss of feeling. That may be why he has difficulty keeping an erection from oral as well. I myself have never found oral that stimulating except from one girl that knew some good techniques. And yes, the head is generally the most sensitive so it feels the best.
You'd really have to talk to him about it. Don't do it during sex, because you might put pressure on him. And don't be pushy or judgmental or make accusations. I know you say he doesn't want to, but he's the only one that knows. And if you're getting married then you really have to be open with each other.
If sex is ever causing problems in your relationship, then you could see a sex therapist. Individually, as a couple, or both.