FTM needing advice

My son is 18 months, he is circumcised. I'm not sure if that has anything to do with this, but he freaks out when I change his diaper and screams when I touch his penis. I've taken him to the doctor and they said nothing is wrong with that area. It's so hard to clean him up because he twists and kicks. I know it sounds like I'm torturing him to other. I'm at a loss. I stop using wet wipes and started using soft wash cloths. That hasn't helped the issue.