gina, can you summarize how anyway? even though I never trained him that, I'm curious

I train with a clicker (and I love it) what I do is wait for the dog to lay down (clicker trainiers call it capturing) and when the dog lays down <click> and give a treat. I throw the treat so the dog has to get up, then I wait and when they lay down <click> and give a treat. I don't say anything I am just waiting for the dog to offer the behavior. The dog my be going from a sit to a down which is ok, once they are relibly going to a down, I start clicking only for a down, not a down from a sit.

If you don't train with a clicker or don't want to you can teach down from a stand by holding a treat in front of the dogs nose and going straight down. This might take several attempts so don't give up. Say "good" when they lay down and give a treat from your other hand, in a way that makes the dog have to get up. Repeat again.
Don't say down until you have the down (behavior) you want. They say once you name the behavior that is what you will get.
I hope this helps.