Have you tried going from a stand?
Start with him standing, then use a treat and put in front of his nose... when he is focussed on the treat, move it down and in - so towards his front feet.
His head should go down to follow the treat - he will then go into a "Bow" position and if you hold the treat a bit longer - his bum should follow.
If he is persistant and will not go down - give a GENTLE push downwards on his shoulders, then once in the bow position you should be able to lightly push his bum down and it will follow.
IMO this also produces a nice square shaped drop from the very beginning - so reduces the need to "shape" later on.
Images added to try and help explain the "Between the feet" luring and what the bow position would be like.
Images kindly borrowed from Google search
