I have a 2yr old that is built like a barrel and still very butt high, my double J #10 tree that I have for my other horse doesn't fit him. Im not gonna start riding him until this fall and I know his back is going to change somewhat, but what fits your guy's mutton withered barrel backed horses?

Charmayne James wide tree. Both my horses are built just like that. However , my Corriente also fits just as amazing!

 Have you tryed a Double J wide fit? 

 Double J flex?

If possible take your horse to a saddle place & have him fitted. I took my wide guy to allen ranch saddle shop & they have different trees so we got the right fit for slyder. 

I have one just like that who I got fitted, a Double J Pro Flex wide fit like a glove.

Have you ever considered a Treeless? I used a treeless with a CSI pad on my colt until he stopped growing, it kept him from getting sore and it worked great until I could get him into a saddle that could fit him. If not, I also agree a Cactus Charmayne James fits those wide mutton withered horses.