Just curious, but how many of you have had luck w/ fixing an uneven front end on a young horse (~6) that's only been jumping regularly for about 6 months? If you did have luck, what did you do to fix it?
The uneveness is usually the same leg and it's not hanging open, but rather, it's tight but forearm not parrallel with ground (knee slightly pointing down, leg tight). Sometimes horse jumps square and it looks lovely. Either way, room to spare between horse and jump (whether jumping square or uneven).
Conversely, how many of you had this issue and couldn't fix it...did you try, etc?
If it is just the one leg? Maybe start with a good sporthorse vet willing to watch him go? May also be a conformational defect that is preventing the "weak" leg from working like the other. At 6, he is not such a baby and, if it's been a regular jumping program gradually stepping up in height (as in more then 2') he should not be doing this. If you are still over poles and itty, bitty crossrails, it may improve when the fences require an actual jump.
Otherwise, small, square oxers ridden to a deep spot, grids/gymnastics and placing poles are the traditional excercises. Look to rider position as well, be surprised how much it can infuence a horse's form.
When opportunity knocks it's wearing overalls and looks like work.
My first stop would be an appointment with a good diagnostic person, whether it's a vet, a chiropractor, or some other type of a body worker. My TB, who did the level 6 jumpers this year, has a similar problem where he almost always jumps with his right knee twisted and down a little further. I know where his uneveness stems from....he has an off-center pelvis that we've been working on since I got him. He's HUGELY better than he was, but it still shows up when he doesn't make much of an effort over a jump. Regular chiro and acupuncture has been the key to getting him straight.