Since the first half of your question seems to have been addressed already, I'm only going to give my advice for the second half. In most cases, when the horse learns to step up under himself, his headset will fix itself. It is because when he steps under himself, he raises his back, which causes his head and neck to relax and go down. If your horse is lacking the muscle for this, the "easiest" (not really easy, but I suppose in comparison to other methods!) way I know of to build it up is lots of down transitions. However, this will only be effective if he steps under himself into the transitions. If, when you ask for the down transition, he throws his head up and hollows out his back, it will not be effective in building up muscle.