can't really say anything about his conformation from the pictures, but I can give you some advice on what types of things to work on to muscle him up.
For my barrel horse, first mistake I made was going too quick too soon. Make sure you have muscule memory from working slow so your timing is spot on.
Heres what I did with my horse...
I did ALOT of dressage flatwork, yeah I know it sounds weird maybe, but for me I like my horses being able to have unnoticed cues. I did ALOT of leg-yeilds, hunches in, shoulders in, all that. Also Id say work on a nice "frame", it will strenghten up your horses back and and help prevent any issues in the furture.
As for that rump!!!... You'll want to get that as stocky and full of muscle as you can, seeing as that's where all the power comes from. Do ALOT of hill work if you can.
I got my horse doing really good with all these excersises, she actually went on to win AQHA IL state in barrels a couple years back. Pm me if you want more info or have questions..