Sounds like the cause of the infection is not important but the pore circulation from poorly controlled blood sugars. That could be why they cant get the infection cured. He might benefit from hyperbaric chamber. A chamber with pure O2 to help wounds heal.
He should see a wound care specialist.
Hi,
the medical team will not consider a TKR when there is infection, as the replacement would likely fail under such condition.
Your father may need to work both with General medicine to get the infection under control and hopefully cured, and also with endocrinologist to get his blood sugar under good control, which will also help with the infection.
The key reason his infection is not getting better may be related to high blood sugars. Normalising blood sugars with insulin and/ diet (low carb typically works best) will be a good approach to help him do as well as possible.