I don't want to be one of those folks that always hollers "call the vet" but if the wound is going backwards in progress, maybe a vet would be in order. ( I read he was getting injections and such, so you've probably already done that, but if it isn't healing properly mentioning it to the vet again would be a good idea.)
I know I always worry myself to death anytime my horses get a health issue. Normally for small things I do treat them myself. But if they aren't getting better I would be wanting a vet's opinion.
I agree, an infection is going to need to drain and that may require it being manually done.
Is there any way you could be mistaking pink tissue for proud flesh?