Trot, trot, trot! Trotting is the BEST thing you can do to build topline.Lots and lots of trotting.
While he is building, do lots of stretching, but not with his nose out - with it in. Then do short period of collection in training frame. Until they build that muscle, they can really only do a few minutes of this at a time, so once he get the idea, you can then start having him stretch and then shorten and then stretch. Until he builds up that topline, he won't really be able to canter collected, so when you canter just work on keeping it balanced, up and consistent. And honestly, the best thing you can do to get a good canter is - you guessed it - lots of trotting! As he gets more and more comfortable trotting collected for longer periods (you'll also physically see the muscles developing), you then will be able to focus on starting to collect his canter a little at a time.
Make sure while you're building that you re-access his saddle fit every couple of weeks. What fits him now probably won't fit his new muscly self and you can set back the development a lot by working him hard in a saddle that doesn't fit.