He is 15 months old and has shed heavily since about 10 months oldThat would seem the normal sort of teenage shed, changing from puppy to full adult coat. Although I would not deny that a dog needs a good balanced food the lack of shedding may as well be a sign of poor health as well as good, dogs should shed as not only is it a way of maintaining their body temperature but it is also a way of shedding dead hair and replacing it with growing hair a normal and desirable mechanism.
Bitches seem to shed in relationship to their seasons where as dog seem to do it in response to heat so a dog kept in a heated house will shed more than one kept in a kennel, in deed, it can become an almost continual process with a dog who lives in a warm home for most of his life.