If the vets prescribed antibiotics he must have suspected an infection, the fact that the medication helped sugggests it is a medical thing. Id get him back to the vets to find out. Westies are prone to skin problems and some skin probs can be diet related, ask your vet about trying him on a different food. What my vet does is recommend a really obscure flavour of food, Walthams do a caplin and tapioca flavour (the important thing is that its flavours your dog hasnt experienced before) but its a total exclusion diet for about 8 weeks, no treats, no cheese, nothing but this food. If the skin problem then calmed down we knew it was food allergy, if not they were sent for referral to a skin specialist. It may not be allergy at all though, it could just be a mild irritation he picked up. Always best to let the vet take a look.