We can help with general matters but when it becomes medical, only a vet can fully diagnose or prescribe medication. I agree with Elusive that they could be tumours that have ulcerated and antibiotics could help the ulceration and infection but not remove the source of it. I have had a hamster with a tumour that ulcerated like that. But we don't know because we're not vets and don't have the diagnostic tests - so they could also be abscesses that have ulcerated and those would improve with antibiotics. Sometimes even vets don't know, unless they take a biopsy, which can mean a whiff of anaesthesia and also be stressful for an ageing hamster. So in my experience, vets sometimes take a conservative approach initially, by trying antibiotics first and if that rectifies it then that answers the question of whether they are abscesses or tumours. If it doesn't rectify it then they wouldn't (or shouldn't) do surgery on a hamster of this age when they cover such a large area of his body. There is insufficient pain relief available for cancer. Metacam may help for a while but eventually it can mean the only pain relief is to humane euthanasia.
It's really good he is playing normally and not showing signs of extreme pain like hunching and not being active.
But only a vet can prescribe antibiotics and it would seem that is the first thing to try. If your vet can prescribe a hamster dose of Septrin that may reveal some answers ongoing as to whether the issue resolves or not. If your vet can't do that, and the fusidic acid externally isn't helping, you could try the herbs I mentioned earlier to see if the sores heal to help him be more comfortable, but that isn't quite the same as treating the source.
Keep an eye on if he is biting at the sores at all because that is also an indication they are in pain - they try to amputate the painful the lump themselves.
