For topline you want to work at the extended trot a bit, making sure to keep him in proper posture, this will help build up his muscles properly to allow him to keep a good frame. When you allow him to hang his head too low and stick out his nose, he is avoiding the effort that it takes to stay rounded, so just keep on him and practice practice practice,.
Some other things you can do to help build him up enerally so he has a better foundation which will allow him to carry himself better would be cantering in a collected frame, hillwork(slow and steady walk builds better than just running up them, it forces him to pul himself, to get his butt under him). Also, though i dont tout using contraptions, using side reins when longeing with just enough contact to get a bit of a flex at the pole, can also help him to build the muscles necessary to have a nice level topline, plus with you on the ground you will be able to see for your self what you need to improve... Extended Trot Exercise
this is an exersize i found that could be beneficial too.
You guys look great, you just need to build a better foundation and you wont believe what an improvement it will make!
Im not a pro or a trainer but I have been working with horses my whole life and these are just things Ive found that help with topline...Hope this helps!