There are plenty of varying opinions on asking a horse to drop their nose and collext, some feel you can push them into the bit, which is true of course, but your horse needs to learn that when you're pushing them into the bit and engaging the back end you are not asking always for more speed, but rather animation and collection.
The nose out will result in a 'U' neck which in the end as G says, a hollow back and it is not all about the cosmetics (of course it looks great when theyre collected!) but if you have your horse rounded they are developing proper athletic muscles. I found that while training my mare to learn to collect and drop her nose the rein really helped to set her mind on what I was asking. I started the whole process at a stand still and that was beneficial. Have him stand, start with try with one rein first, but sometimes using both helps ... pull up and hold gently until they give and release immediately. The slightest give deserves the reward when training this (as he starts to understand, stop using both hands (if you needed to do that) and rather use your outside rein to steady him and prevent her from turning his head in and cue the nose drop with the inside rein)
Once they understand 'dropping the nose' while standing still you can then ask him to drop their head before moving out. So cue the dropping of nose and then ask them to move forwards. Once they understand that practice them acknowledging the rein while you are walking, start moving out collected and every time the nose goes up and out again ask him to drop. It is a whole new balance for a THW to move collected and so you'll find yourself asking repeatedly for collection until they get their balance.
I spent months on teaching my mare to stay collected, it is about more than the nose being dropped, but engaging the back end and staying in balance. You'll maybe find that when you have his nose down he tries to step pace or fox trot, this tends to happen because the rider isnt engaging the rear of the horse and unless you are motivating from the back while asking for them to drop their head they will get strung out in the back to compensate for the front. I'm not sure your level of riding or experience and some of this you may already know so if you do, disregard :) , but it definitely sounds like the horse was not taught to collect right from the get go, so as far as changing the habit, I would go back to square one and make him learn to do this at a stand still and move on from there.
Hope this helps some. Goodluck!