Hunters bump?

My three year old Red has for some time had a noticeable dip between the spine and the pelvis/hip. Since he's not doing any work at all, I haven't thought much of it really. I figure it will re-adjust itself when he rolls.
Of course, Red is far to prim to do such a thing as rolling...just wait till I dump a bucket of water on him...

Is this something that I should be really concerned about? He doesn't seem to be in any pain. It's just really...strange.
He does have a serious lack in muscle in his hindquarters and top line though. It's been a constant battle trying to bring up his weight since I got him

I intend on starting his exercise program quite soon. I won't if it could cause further damage, though.


He has also taken to jumping the rope I use to separate the paddock from pasture during feeding time. It's about 2 1/2-3 foot off the ground. I'm going to be adding a second rope for both his sake and Sam's. He's only done this maybe 3-4 times, but at his age I don't want him jumping what-so-ever.

I don't know, pictures would definitely help. I am tired and having a bit of trouble visualizing what you are talking about. :)