Sorry for the double post...
One thing I will add in topline comes with your horse using it correctly. Since he's got really nothing going on I would start with it on the ground, with belly lifts. There's a whole thread I made about stretching and muscle building, with links to really cool stuff.
Stretches (and muscle building) for your horse
Hope that helps, once he bulks up and gets to a good weight and has a nice topline, I'm sure you'll be able to do anything with him :)