Have you got a video? That would be very helpful. Is he gaited? Is he doing a gait? Is he perfectly sound? Honestly with my MFT I can't tell half the time if he's lame or if he's in his funny gait. I would check 100% soundness if he's not gaited. If it's not lameness or gait you could also check his back pain/saddle fit, painful back or improperly fitted saddle can cause a horse to shorten or lengthen a stride, with one or all legs. I'd also check bit contact, are you holding him back from moving forward? There are SO many aspects to check a video would be helpful.