In the last 3 months my 5yr old male boxer dog has developed a bald patch about 60mm by 60mm on his back We thought it may have been food related so have changed his diet, he is a fit lively dog, the vets have checked him out and said it should grow back but there is no sign yet that its even starting too.
Any suggestions.

Seasonal alopeacia, in which case it will clear up on its own as the weather and sunshine improves or, if it persists, I would get your Vet to check his thyroid levels. If it is the seasonal alopeacia it may return each winter. Melatonin is the boxer fraternitys recommendation for treatment. You can buy it online.