Is there any way he could be walked in a totally different area for a few weeks to see if that makes a difference?
I also wonder, if the lumps go down completely at the end of a course of treatment? It could be that he just needs a much longer course of antibiotics. If there is still some inflammation/infection sitting there at the end of a course, it would obviously flare up again once the medication is stopped. AND he will be building resistance to each antibiotic too.
Yes, Hibiscrub is an antiseptic. No, it would not treat a fungal infection. I think you would need antifungal tablets and/or cream or shampoo.
I am sure your vet would recognise histiocytoma. Unless it was atypical in some way?
Is he in pain with these lumps? Does he get a raised temperature at all? Can you describe any smell coming from the pus?