Medication causing issues?

My boyfriend is on medication due to chronic health problems that he has. He has been on them his whole life. We started dating a couple months ago and want to progress our relationship. I was about to give him a bj but then it went soft. He says he thinks it is due to the medications that he has to take. He says that sometimes his you know what doesn’t cooperate at the worst of times. It doesn’t happen all the time but occasionally it does. Has anyone else had this experience? Any tips?