Back when I first started to take obedience classes, the instructors suggested this way to reinforce certain exercises. This will REALLY help with your down-stay problem. When it's feeding time, ask him to do whatever exercise you want to work on ie: sit, stay, down, down-stay or even stand. Start it out slow - for a minute at a time - when he performs the required exercise for 1 minute, he gets to eat. Next time, try 2 minutes, etc. If he doesn't do as asked, start over. He WILL get the idea. Also use whatever command you want to reinforce BEFORE letting him out the door for walks, potty time, etc. This was suggested to us when we were working on the timing of the exercises for the obedience ring - really worked well. I still use it.