As someone else said, check your saddle fit. If you do not know how find someone that does.
If the saddle is OK then I think you have a horse that does not have the proper musle developement for gaiting and he has also figured out how to be lazy and get away with it. All of gaits you say he is doing are easier than racking. Dog walking is a four beat gait. Do it a lot, but pay attention to what the conditions are. Keep him from going in a straight line. Doing cirlces, serpentines, going over 4x4's, up and down hills, tall grass and sand are what you are looking for. He will have to use the muscles for a 4 beat gait doing these training exercises. Do not go any faster than he can go and stay in a four beat gait. I dogged walked a TWH 1.5 hours a day for 3 months before he could do a flat walk. It took 9 months before he could go wide open racking. He now racks to the barn at feeding time. If you are not sure if he is doing a four beat gait take him on a hard surface and listen to him. If is 1,2,3,4 not1,2...3,4. Take him out on a hard surface with a horse that does rack well. Your horse listening to the beat of the other horse will help him. Beleive it or not I have seen this work more than once.