Another way you could get him to do a down would be to lure him under your leg. Just sit on the floor with one foot on the ground. Your leg should make a triangle. Then, try and lure him under your leg with a treat. You probably won't be able to do this in class but you can practice this way at home. As soon as he's completely on the floor, click him (if you're using clickers which I assume you are), and then treat him. I would also make a huge deal over him. I wouldn't pair it with a word just yet. Let him get used to the movement and then, when you think he's ready, say 'down' as he's in the process of going down. Then click and treat him when he's completely on the floor. Eventually you should be able to phase out the leg and just say 'down' and he'll go to the ground. Good luck! Once he figures out what you're asking of him I'm sure he'll pick it up pretty quick. And they are very distracted in class. Good luck!