Penis pain/discomfort during/after orgasm from oral sex

Hi, never done this, but recently I have been seeing discomfort in my husband s face while he is receiving or completing an orgasm during oral sex. I dont want to cause him pain, and I asked him if it hurts and he said yes, but I didnt want to intrude and ask what hurt, when and well you know... All I know is that it hurts him and I want to know why or what it can be, and what I can do to help not hurt him. We havent had sex in over 4 years. Niether of us had any other sex partners. Is it because He hasnt had an orgasm in so long? Or is there another problem? We havent has sex yet, we are just fooling around, getting to know eachother again but I want sex to be pleasurable to him... not painful! Please help me understand, much appreciated!