Hi. I Have A Question That Im Having A Hard

Hi. I have a question that im having a hard time finding the answer to. Me and my husband have been together for 4 years and never had problems in the honeymoon dept, to put it nicely. Here lateley its been different. his genital area is fine untill after we have sexual intercorse. This stated roughly bout a month ago. He gets these white little blisters all the way around the top part of his genital area. It goes away after 5 days but hurts him. We tried again having intercourse but the blisters pop up again, so we just stopped. We dont sleep with other people so hes thinking since i have my tubes tied, maybe its a hormone inbalance or something. what could it be?