This is just a critique of the pics. First, I think you guys look really great. Don't be so hard on yourself.
Joe is just beautiful, a typically-built TB with nice legs and nice beginning muscle tone. I love his face!
Your lower leg looks awesome... your heels are very nice.
As you've said, your back needs some work and believe me I know how hard that can be, as that is my major problem as well. I would try riding on the lunge line, if you can, without reins. This really really forces you to sit back and let your legs do all the work. I've only done this a couple times, as its hard to get someone to lunge me, but it works wonders.
Someone mentioned flexing at the poll, and yes he needs to bring his head down a bit and start "accepting" for you. But I really don't think he's a long way off. He is still young and I already see a bit of flexion through his neck.
I would work on your hands too. In some of the pictures, they are uneven and unsteady looking. This often makes horses confused, as it unsteadies the level of pressure and also hinders with direction. I would loosen up your reins and work him nice and low, working him through his back legs, up into your hands (contact).
Overall, you both look very nice together, a great team!