It's definitely hard to tell without a full body picture, but It does look like your horse is holding his head in and your hands look like the may be low? So, too much hand and not enough leg/ seat.
To get him working over his back, I suggest thinking "up and out" with your hands. When he ducks his nose in, lift your hands, add leg + driving seat, and let him bring your hands forward. Reward any small improvement. I also suggest doing this in small durations, then allowing him to stretch his head down and out, which builds the trapezius muscle before and behind the wither.
Don't worry about him going above the bit, as It is quite normal for a horse to do that until they have developed the proper muscle to hold their head in the proper position. It can take up to 1 year for them to really develop that muscle. Many people make the mistake of making a horse keep their head down, which results in tension against the bit, rather than a horse that searches for the bit. Getting a horse straight and on your aids is also a big part of getting a proper 'frame'. Most horses will naturally go onto the bit when set up properly and no horse will naturally relax when crooked or unbalanced. If he does go above though, you can lift your hands up higher and send him forward until he connects with the bit again, then bring your hands down and forward with him. When a horse's head goes above the bit and your hands stay at the same level, the bit now acts on the bars of the horse (as you can sort of picture below). Raising the hands allows you to keep the bit action more or less on the corners of the horse's mouth.
Rollkur, is when BTV is taken to an extreme and they are different. In rollkur, A horse's nose will be taken in farther to his chest and held there for an extended period of time. Greener horses or horses lacking muscle will sometimes dip behind the vertical from time to time due to being tired or inexperienced. This puts more weight on their front end and allows them to avoid using their hind end properly.