Pamela, I did it almost exactly the same way with Tamarack. When I started to ask him to rear, he would fling his head in the ugliest attempt (Jesse, he was really FLINGING his head and not just raising it), so I stopped asking for it and decided instead to use other behaviors he already knew, to help strengthen the haunches - to ask him instead to sit back and learn to bend his hocks. He already knew how to lean back from our training the bow.
I decided that what we should do is to work on a very correct levade or pesade...whichever comes first is fine with me. So we started working on sitting back on the haunches, and after a couple of months his front feet will occasionally leave the ground...but leaving the ground at this point isn't the goal really...it's just working on his muscle memory to set himself up properly, to bend his hocks and sit into a lightening of the forehand without any excited head flinging.
I have used a combination of three behaviors he already knows. Two of these behaviors are the key to almost all the collected exercises...ramener, and Goat On A Mountain Top. The third behavior is to lean back when I ask. I also use my hand on his chest, or I use the cordeo in a soft back and up motion.
So Jesse, you can work on all these things. With such a young horse, please, please take your time. If he offers to rear freely, take it gladly, but for weighting the hocks purposely, take it slow. Simply teach him for now to allow himself to be leaned back. It is a matter of trust - just as much as a horse allowing us to lie them down or to bow - if they willingly allow you to manipulate them in this way, then you have the cornerstone for rear whether it's levade or pesade. Both must be performed calmly.
You have loads of time with such a young horse, so just see where it goes! In liberty work, each horse chooses his own favorite exercises...and the fact that you have so many horses to play with is enviable to say the least!
All of your work in lateral movements, and in the lengthening and shortening of the gaits, will of course also help with strength and muscle building - which will also help result in the lightening of the forehand, offers of suspension, etc. It's ALL good!