What a pretty boy! He's got a nice back and slightly sharky withers. If you're interested in used, I have an old 17 31' Stubben Parcival II that might fit him nicely. I used to use it on a grey mare I was training. It has a nice forward flap and very flat seat and narrow twist on it and is in beautiful condition. Gave me great position. I'm only asking $500 and would be willing to send it to you to trial so long as you paid shipping both ways if you returned. Let me know if interested and I'll get pics for you.
Hmmm...a Bates or a Collegiate might be worth a try. Although he seems a bit narrower then any horses I've seen them used on, the interchangeable gullet could help with that...the general shape of his back seems similar to one of my coach's mares that she uses her Bates on. :) Both saddles come with long flap options too, I'm pretty sure. There's usually tons of both around, might be worth a look!