worrying habbit

[Tony Mc Carthy]

id just like to ask if anyone can shed some light on a problem iv had with my 18 month old rottie...at about 10 months old he started chewing away at his frony forepaw, just below the elbow joint, to the extent he exposed some bare flesh..i did visit the vet who perscribed some cream and a lamp shade for his head. the cream did clear up the wound, however he has recently started to do the same thing again...He is up to date in worm and flea treatment etc