This is really interesting - George was recently vet checked as I was pondering arthritis and he's getting a blue glaze to his eyes. Vet said yes, his vision probably is starting to go now, but he does have arthritis in his knees and it's more likely to be this which makes him pause at the guinea pig steps before he gallops up. Now George is flipped over every night (impaction check) and I'd noticed more skin underneath than fur recently but just put it down to him being fat and sweaty (close your ears, George). This comment has suddenly made me realise about the third thing she said which was that for some reason he had a lot of hay and hair stuck in his bottom incisors (down in the little gummy pocket) and that I should check his teeth regularly. All that hair could well be from having a go at his knee area and thus explain the balding patches!
To my teen's horror I also asked quite an explicit question about why his boar bits are hidden away inside his anal sack and whether this was a common arrangement and lovely vet hesitated before she said, "well... he is a bit fat..." so who knows what might happen when his meds kick in! She said on the way home, "OMG- why did you even bring me?!" and I told her to make the most of a trip out, but also that the car could get very hot in the back and the pigs couldn't tell me so she was sort of like the canary in the mine because she does go visibly pinker.
-----------------------------------------------------
Sorry - just realised I hijacked your thread there! I'd stick with the xeno (I've always found it very reliable and not had any side effects) and see if the hair starts to regrow. I'd also ponder the barbering possibility if he's an older fellow. Finally I'd also think about how piggy lies down and whether he rests on this area of his legs as he lounges. Could there be something he's lying on or in that irritates him? eg if he has fleecy areas could it be irritation from a biological wash powder or fabric softener (I find it best to use non-bio and just white vinegar in the rinse) Or if he's on shavings does he lay in his own pee (and I'm looking at George again) and maybe cause himself a little problem like that?
Good luck Carlos you cutie, and I'm so pleased that you made it through your dental surgery x