Thanks guys, the vet examined him and there was a spot between two pads that she could see was sore and weeping so he's on antibiotics and anti inflammatories and a cone of shame, and goes back Thursday for a check up. It has been a week since he first started chewing it (he went to vet on the thurs) and he is still chewing it when I take the cone off - but not always, once he starts he can't stop.