Most men are only a little sore for a few days. Cold packs and Ibuprofen should do the trick. You're concerned he might be resentful later: If he made the decision to do it, there should be no resentment. Usually it's the women who spend decades being responsible for birth control, going through pregnancies, labor and delivery, breastfeeding, pelvic exams, etc. Nothing wrong with men doing their part.