What Causes Pain In Penis During Erection?

Hi,my husband and i were both virgins when we married. we are 4 years married and only in the past year are we have gotten completely comfortable with sex and all aspects of it.my husband, though, has only been "rock hard" maybe 3 times since we got married, and each time we havent had sex because it hurt him too much. and we couldnt put a condom on because it hurt too much.and then the rest of him goes soft (because he is feeling pain i guess) but his foreskin wont go back to cover the top, and he is in pain for most of the night until his foreskin covers the top again.why is he in pain?and how can we over come it? i would really like to have sex with him when he is rock solid, because when he is only half way hard then his penis kind of flops out of me quite alot and i get very frustrated. it also means we cant try very many different positions because he is too soft and would just flop out.help!! :)