Rather than equitation (although it is important), we need to analyze our riding pictures for function. Although your basic eq is pretty good, you need to be more of a 'rider' and less of a 'passenger.' Now, this is all just from one picture, but in this shot you're very much riding 'front-to-back,' when the opposite is what you want. What I'm infering from this picture is that you want your horse in a 'frame' (hate that word.), but you haven't taken the necessary steps to get that nice, round outline we english riders like so much. If you take a look at the training scale:
You will see that collection - or, the time when roundness and on-the-bit comes into play, is the very last factor. Everything else needs to be in place first. What I'm seeing is a lack of hind end engagement, because the rhythm and impulsion was sacrificed in exchange for a pretty headset. There is also limited relaxation, because of your tight hold on his mouth. Roundness needs to come from the hind end - and consequently, from your legs. It would do you and him well to do a lot of loose-rein work, to give you the feel of adjustability exclusively through your seat and leg, and him the chance to really use his hind end and start working over his back on his own. He'll figure out the balance better without help. Right now it seems like the reins are your lifeline - it doesn't look like you would be confident to get rid of them and ride without his mouth. That's not the case - once you do a ton of on-the-buckle work, you'll both be much more responsive to one another.
Basically, once you've got the hind end, you can start to work on riding into a soft half-halt up front. Between half-halts there needs to be a 'give' - basically a steady outside rein and elastic inside rein to encourage him to accept the bit. It may take a while - he may be evasive at first because I think he's used to an overly tight rein and a twist-and-pull flexion to get his head down. But putting the time in will definitely be worth it.
Sorry for the novel - I always get carried away with critiques ;) But it's important for us to ride our horses correctly and sympathetically!