The best way I've found to train for down is to hold a treat in your hand (the stronger-smelling the better) so that the dog can not get to it but knows that it's there. Since your dog already knows how to sit you can start in that position, then give your dog the down command while bringing your hand with the treat in it to the floor at his feet. He may wiggle around a bit at first, some dogs stand up and then kneel down trying to get at the treat, but eventually he will try laying down. The moment he does, praise him and give him the treat. It generally only takes a few repetitions before a dog figures out exactly which position is getting that reward. Remember to only give the command once, then wait until the dog gets into position. Repeating the command may confuse him at first, and later may make him think that it's okay to ignore you the first couple of times you say it before complying.