14.1hh, 5'6" or 5'9". He has a proportionate back length, but a disproportionate belly depth. If the rug is the right length, I sometimes have trouble doing up the belly straps.
My really narrow weedy 14.1hh Arab took 5'3 and at the other end of the scale our 14.2 heavy weight show cob took a 6'6 but I expect a 'standard' 14 hand pony would be in a 5'6. Much depends on whether your pony is of average build?
My 14hh Arab X Welsh B takes a 5'9"; she has a broad chest. My 14.3hh TB x Connemara takes the same size, but she grew a big butt this year so we may be looking at new ones...