You shouldn't shave a dog with a double coat. If he's shedding big chunks of hair, he should be brushed, not shaven. The long hair actually protects the dog from the heat (and cold).
So, take your brush instead of the clippers.

I actually use a rake (made for brushing dogs) and a wide-toothed comb. Or better yet, inform your brother that he needs to take care of his dog by brushing him.
Also, make sure he (the dog) has plenty of water to drink, lots of shade and even a kiddie pool with 6 inches of clean water in it to cool himself off if needed.
