how about a treeless? its easier to get a dressage saddle that would fit him in treeless and you can have a bit bigger seat if it suits you.
I had a trekker dressage saddle that fit a very short backed welsh b, he was 13 hands and fit into susies 5ft rug, tho he may have got a away with a 4ft9 . The saddle was about 16inch underneath, but the seat was more a 17 1/2 inch, tho could be made a bit smaller.