First, the trot poles are GREAT for building up Solo's muscle, so definitely keep using those whenever you can.
I would suggest you try a little bit of riding with no stirrups. Just place them over his neck right in front of the saddle and practice your leg position at the walk without them. Remember to hold on (gently, not tightly -- don't grip or anything) with your thighs and your calf, while pushing your heel down -- but don't lock your ankle. I don't know if you know what that feels like or not, but once you do, you'll feel the difference. When you lock your ankle you're simply pushing your heel down (and that's usually how you get your heel REALLY far down). What you should be aiming for is to extend the muscles in the back of your calf to drop your heel down slightly below your toe. Also, remember to try and keep your toes pointing more forward than out.
At the walk, how fast would you say Solo is? Does he just kind of plod along, or does he have a fairly quick walk? That will make a lot of difference for what I suggest you do with him at the walk. And I'd rather not type out both sides, so please let me know!
Now, at the trot, I'm going to suggest you pick up your stirrups, and ask for a nice bold, forward going trot. I want you to get him to that feeling where it feels like he's just about to canter, but not quite cantering. And I want you to ask him to really move off your inside leg. So you're going to apply a little bit of pressure with your inside leg as you go, and slowly start asking for a bend. First thing you're going to do is keep him on the rail. Second thing you're going to do is just ask for a little bend. You're going to do this by pushing ever so slightly with your inside leg, and keeping contact, to maybe even giving a little bit of a tug with your inside rein. Trust me, you will feel it when he steps under himself and drops his head. (I'll see if I can find some pictures to add of a horse bending properly after I type this up).
The only thing to be careful of when you start asking for the bend, is keep an eye on his OUTSIDE shoulder. Some horses evade really bending by popping their outside shoulder out, and thus avoiding contact in a way. When a horse is properly bending their ribcage should be pressed slightly away from your inside leg, and slightly against your outside leg, their head should drop, and their hind legs really move under them. It usually takes someone to point it out for a while, but you'll learn what it feels like after a while, and it gets easy to spot.
As for the trot poles, really keep working on those. They're great for you and him. I would suggest doing pretty much what you're already doing, except try to relax as you go over them. Just give him a little push with your legs as you get up to them so keeps up a nice, impulsive trot over them, and then try to distribute your weight through your calf and thigh, not just your heel, keep your back flat, your eyes ahead, and your elbows bent.
Now, to sum it up, the things you should really work on with YOUR seat are:
Try to keep contact (though nice and giving contact) with his mouth. Adjust your reins to the point where your elbows are just in front of your hips, and really let your arms move with Solo's strides. Let your seat move with him as well. Relax slightly, shoulders back, don't arch your back. And remember, thumbs towards the sky!
Now, I have a question for you. What sort of work do you do with Solo at the canter, and what does he do when you canter?
Oh, and just a couple more things if you want my help... >.> My computer is currently broken, this is my parents and it has dial-up internet. So if you upload any videos I'd have to go over to my grandparents to watch them, so I couldn't necessarily review them right away for you. Also, if you want me to watch any videos for you, I'm not allowed on youtube, so if you could upload them to photobucket or something that would be super awesome.
And if you want to know anything about me or my level of riding or experience or anything, feel free to ask. =) I can't supply much in the way of recent pictures because my computer is busted at the moment, but I could share some old ones.
Anyway, I don't know if you want my help in particular, but I'd be glad to be your online trainer if you like. I've worked with muscling up and teaching a LOT of horses how to bend with my trainer over the last year. So, I have a fairly decent idea of what I'm doing. =P