Hello Welcome to healthcare magic. First of all I do not think its herpes infection. Herpes starts with small fluid filled blisters which combine to form a single blister which than rupture to form a ulcer. Genital herpes mostly present over glans penis. You can go for Tzank smear testing to confirm herpes. As you mentioned that your husband have diabetes, the rash is most likly to be a fungal infection like tinea cruris ( ring worm infection) for which I would suggestion KOH mount testing. Most common site for ringworm infection is groins. It first starts as small red ring with associated white scaling which gradually progress in size. Pustules, blisters can also present over the patch. Keep blood suger level well controlled and get KOH testing or send me photo of the rash Thankyou.