What kind of brush are you using? You should use slicker brush to remove the dead fur. I've seen people using normal bristle brush which is just meant to "dust" the hair off from the upper coat. If you want a brush which can remove the hair from the inner coat, then slicker brush is for you. You need to brush him for about half and hour everyday for about a week to reduce the symptom.
Is he loosing hair from all over the body or just from specific area? If it's all over the body, then grooming will help but if it's from specific area, please visit a vet.
Could you also start a good hair supplement. This will ensure the new hair that are yet to come out are healthy and shinney.