look like a collar issue , is it on all the time? I try and keep my dogs flat buckle collars off when we arent out walking but want something to grab them by if needed { when people come to door or is the gettin rough playing ect} so i keep a chain martingale on them and there is no rubbing like the collars do. or as people above mentioned a sof collar might be a good option. doesnt look mange to me it looks like a rubbing issue , or it was hard to see there but does he have loose skin that kinda folds there? could be a moisture thing as well , we used to a have a wrinkly dog and the folds would get red and raw looking if moisture was in there { we had to make sure he was dried very well after baths and even playing outside whether it was wet or if very hot and sweaty} or he would get skin infections.