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.