I stick to my previous statement and say that he is a little too skinny, but its great that you've bought him and you're putting weight on him. If you would like to see good weight gain results then I would suggest getting a supplement made by Nutrena called Empower Boost. Its awesome. It puts weight on them better than corn oil but it won't make them hot whatsoever. I love it.
To build up his rump I would suggest doing a lot of backing from the ground, but make sure you're feeding him to keep up with any exercise routine you put him on. I like his front end a lot, it just seems like he needs some muscle toning. He's a pretty boy all together though, just needs a program. :) I'll bet he'll be a totally different horse in 6 months.
Four things greater than all things are:
Women and horses
And power and war. - Rudyard Kipling