I had a mare with a back problem some years ago. With careful manipulation from a very experienced equine osteopath we got it right. It's not an overnight thing though, it did take a long time and this mare was conformationally rather long through the loin. If you have had your pony for 4 years with kissing spines I think you would probably have discovered this before now assuming you have had the pony in regular work.

He could of course be suffering from something unrelated such as a developing spavin or degeneration/bony change in a leg joint which is making it uncomfortable to get his hind leg underneath in canter.

I should have a chat with your vet and get him to inspect your pony, literally, from the foot up. Also, if your farrier is around get him look at the wear on your pony's feet/shoes as this can offer some indication of how he is using his limbs (ie is the wear evenly distributed)

Hope that helps, let us know how you get on