As this is a recent change in your dog's behaviour, it's always worth getting your vet to give him a thorough check up as it's possible there is a medical explanation for his growling - maybe he has some pain somewhere or perhaps a sight problem. Once you've ruled out physical causes, you could consider consulting an experienced trainer/ behaviourist who could observe your dog & give advice on retraining. If you have a look at
www.apdt.co.uk, you hopefully will be able to find a trainer near you. Out of interest, can you remember any specific incident which may have triggered this behaviour? Did something or someone upset your boy at about the time the behaviour started? If something frightened him, then the growling could be a defence mechanism rather than out & out aggression.