Stop (down) command

Sounds like you're trying to do this from too far away. One approach is to have Down while you're walking. That way, you're right next to him/able to guide him/stop him with a hand motion into a immediate down and here's the important part.....treating/praising WHILE IN THE DOWN position. Once you have the close work done without movement then move on to random Drops as he's walking through the room (but, again, not too far away....no more than 8' away, with no forward movement. Then move to 12', 15' 20', etc.

The way I did this for Wally was to put him back where he was gently and then I'd go back to where I was and give the cue again.

If he didn't take any steps and just went down, he got a reward.

I started at short distances, say a couple feet at first and then worked for some longer ones until he got the idea that the cue just means go down - not come back to me first or anything else.

Dogs have trouble learning distance and duration at the same time. Work on the distance first, which will build in duration by default. Put him in the down, take one step back, one step forward and reward but don't click (clicking, or marking the behavior signifies the END of the behavior, which is not what you want in this case). Make it fast, like you are attached to a bungee cord. When the dog can stay down for 10 times at one step away, consistently, then move two steps, then three steps, and so on. Make sure you use your release command so he/she knows when it's okay to get back up.