He is really pretty
While he is thin, he isn't skinny.
Hopefully when you get him to the new barn, they will have a more balanced diet for him. Oats and a roundbale, when there's no grass, is a bit shy in the vitamin/mineral department in my book.
Those high rump bones come standard on certain blood lines until they get older and even if he's kept at a good weight and in condition you may always see more bone than you'd like.
My Generator-bred TWH is like that. Right now he really needs to lose about 40#. He has a belly but you can't see his rump bones. Like you, my weather stinks and nobody's seen a bridle all summer.
If I can get just a little riding time on him, he will drop that 40# BUT the trade-off is that I will see a bit of his rump bones. He's now 16 years old, so that is not going to go away.
Post #13, the second horse (Rusty) is the horse I'm talking about. This was taken Labor Day, 2010 when he didn't have the extra 40 lbs on him. He is 15-1/2 and still lanky with big rump bones because that is who he is. You can see the other two TWH's are much more stocky built. Let's see you Gaited horses
This guy has always been very lanky and very very athletic, where my other two TWH's are a lot more stocky and have been mistaken for Morgans or Quarter Horses from time-to-time. They never did have those protruding rump bones - even when they were young.
Point-being, don't worry so much about that but, when the vet comes ask him about the possibility of ulcers, since those seem to be getting more common.
Have the vet check the horse's palette height and tongue thickness. No particular reason for that except you might just as well check to be sure his entire mouth is ok
He is beautiful. I hope you enjoy many many years together