Goodness me, he looks like such a gentleman! I bet he has a good temperement, such a kind look in his eye, you are a lucky girl.
He looks pretty well put together for the whole, just looks like he needs some solid work. I can see the difficulty he has in being more round and if he is the type of horse I am imagining him to be, you could probably try leg work until your little legs are numb and he would still be pretty relaxed about the situation!
Why don't you try lunging him with side reins? Two reasons for this:
1.) You don't want to de sensitize him to leg signals any further so try your luck on the ground with a lunge line, voice commands and of course a good lunge whip.
2.) When a horse collects nicely and transfers some weight to their back end, it feels different for them. It seems illogical but it is actually the FRONT legs that are the strongest (despite the back end appearing powerful). This is because the front legs are designed for weight bearing as the carry the weight of the head and neck as well as the body. The back legs are designed for speed i.e. taking off down the prairie to avoid predators.
So my guess is that your lovely boy would benefit from making this transition to his back end being more supportive and weight bearing on the ground. This will build some beautiful muscle along his topline also and you will have yourself a horse pretty much beyond conformational critique!
All the best!
All horses deserve, at least once in their lives, to be loved by a little girl.