ive recently changed to a hanging cheek snaffle, which worked wonders for the first month, then i went on holiday for 2weeks, so he had no schooling, came back and we've gone back a gear!!
Lots of transitions to get him engaged and lift the forehand. I wouldn't do any lateral work or reinback at this stage. If the basic forwardness isn't there yet, then you can't expect your horse to do more advanced things. Look up the scales of training - it will help you understand what I mean. First things' first - get your horse going forward from behind and "catch" that power in front with a good contact. Secondly I would get myself a good instructor (or that could actually be first thing to do).
Mostly it is all about teaching the horse to trust the hand. If he is used to a lot of hand this means that you will have to do weeks and weeks of riding with only the leg and NO hand at all, so the horse will have its head up in the air a lot at first. Try and concentrate on riding entirely with the seat and not touching the mouth at all- just having the lightest of contacts- like a feather- do loads and loads of transitions , bending the horse around the leg, using the leg and seat for trotting and walking... and never touching that mouth. Eventually the horse will relax and soften and seek a contact entirely of its own accord and will be carrying itself rather than you carrying him with the hand.
For this to work you do have to be VERY strict with yourself to resist the temptation to niggle the hand and it may take a good few weeks of the horse having its head in the air before the horse learns to trust the hand and seek a contact by itself when you put the leg on.Then the mouth becomes like silk.
Usually a foamy mouth is a sign of a soft salivating mouth.
Quickest? Maybe... economically viable... probably not. And what would you do with the leaning horse? Sell it? There are scores of posts on HHO by people who are struggling to sell very nice horses, imagine trying to sell one that leans.
Anything that involves more leg like leg yeild, shoulder in etc. Also transitions but keeping the leg through the downward transitions on and making sure your upward transitions are sharp so you take the hind legs with you.
Thanks guys, hes been a star up until the last 2weeks (typically i've got the david broome dressage champs in 4weeks!!argh) I think for now i'll work on all your suggestions, lots of transitions etc, and then perhaps after the dressage champs i'll have time to do what seabiscuit suggested, because that makes a lot of sense as well.