Stretches do not build muscle. Were that the case, I would much rather 'stretch' my abs then do the tons and tons of situps and ab exercises. ;)
Putting your horse into a frame really only works their neck, or worse, works the UNDERside of their neck because all they're doing is pulling. What you need to focus on is correct long and low work (a description I wrote here
if you're not sure what it should look like or how it can be done), and then transitions. Correct transitions! Ask your horse to trot-then-walk. If your horse just collapses on their forehand and you lose all forward motion, this is a BAD transition! If this happens, ask your horse to trot again immediately (do NOT punish your horse. You have to show him the right way to do things, not punish him when he tries to figure it out). Same for the walk-to-trot. If he labors into it and plod plod plods into this lazy trot, push him forward! Slow will come later with collection if you want it, but first you need impulsion (relaxed forward motion).
Lots of walk-trot, trot-walk, trot-stop and walk-stop transitions in the beginning--don't worry about the canter transitions for a few weeks because if you want correct ones, he won't be able to do it until he builds the muscle.
If you focus on the long and low, your horse will put himself into a 'frame'. If you show them how to do it, and you let them do it in a relaxed manner, the horse will relax at the poll and drop their head, and reach for the bit. Putting the horse 'on the bit' or 'in a frame' has nothing to do with pulling the reins, or pulling the nose under the poll--correct contact has do to with the poll moving over the nose
and the horse relaxing at the poll to allow this to happen. Then the horse will search for the bit and hold the contact
It's a completely electric feeling. :)