OK so another update (I love being on break and having a better situation where I can work Flash daily!). Today, we mostly worked bareback on stopping, side passing both directions, and backing. Though I was afraid side passing more would encourage him to do it more often when I wasn't asking for it, it seems our lesson with reinforcement of sidepassing the correct direction on cue and also reinforcing stopping and holding still really stuck. Like the part Tiffany pointed out, I really do think the extra work taught him to pay attention to the cues I give him. Yay for stimulus control!
Anyway, he's now sidepassing BEAUTIFULLY to the left and always goes the correct direction. Unfortunately, we had some trouble to the right now, so we'll need to focus on that a little. It took me a minute to diagnose our problem, though. He wasn't as coordinated as I'd like and was going forward or turning as he was side passing, and I'd try to adjust him. But, he was getting confused with my adjustments. As it turns out, he was reacting to my bum moving! When I taught him the cue to side pass, I was giving him a spur and dropping my hip on the opposite side to push him over as well as open up the direction I wanted him to go (i.e. to sidpass right, I gave a little spur on the left and dropped my left hip, shifting my weight more to the left). As with our initial work when I was teaching him to ride in the first place and really emphasizing the use of my leg (as I've always been a hands-only rider), it turns out he connected my seat and leg cues to what I was asking him to do rather than just the reins. In fact, once I corrected myself and made sure I was keeping my weight to the left even while holding him back with the reins from moving forward or asking him to keep his front and back together, he immediately responded correctly. In fact, I he even side-passed when I dropped the reins altogether! He's making me a much better rider!
Afterward, we did some leading at liberty just for fun. I ran around the arena and he followed (he got really excited after a bit and began loping around with me), then we'd stop and I'd ask him to back up with me. He was nickering bunch each time he heard the click and even dropped (which by the way, he ALWAYS drops at least a few times when we're doing clicker training and occasionally "stands" straight up if you get my drift - have I mentioned how much he enjoys CT? silly geldings not knowing what that's actually for.). We did a few yielding exercises at liberty as well, including the pivoting we've been working on and he was a superstar. We have so much fun!