Vet thinks it was a small scrape on the hock itself. YO thinks Amber may have trod on him (she is very clumsy like that) and if she had something nasty on her hoof, it could have got in that way.
He could have got something from the field but I would have thought that would have shown up sooner and would have had got into his blood stream and so would have shown up on the bloods.
Next stage is a course of antibiotics and hopefully that should clear it up.