Dogs shed worse at different times of the year......I think dogs with thick undercoats like that "blow" their coat about twice a year. That's probably what's happening with your dog right now.

Just keep brushing and brushing, the shedding will eventually slow down again. How old is your dog? If he's younger, he may just be getting his full coat in, which will obviously shed worse then the coat he had as a puppy. What kind of brush do you use?