Building a horse's muscles up merely takes time.
Walking up a steep hill is good exercise. Just make sure you are sitting correctly and if necessary make life easier for the horse by taking your weight off the horse's rear end. Walk up the hill, then walk down.
One day introduce the trot. Short bursts at first , then longer bursts as he gets fitter. Listen to his breathing. Feel the strain on his legs.
Don't trot downhill- go back to nice steady foot placement at the walk.
Canter - yes when the chap is fit and the ground is suitable. But only until he starts to hesitate, then trot, then walk. No cantering downhill - yet.
A horse is not a machine - you don't put him into first gear and then press the accelerator. Listen to his breathing. Feel his exertion. Let him walk, if that is what he wants to do. Feel for him.
If he comes home a little warm OK, if he comes home with a little sweat - fine. If he comes homes sweating and puffing and lathered up, then that was too much and you should have known better. Your fault for letting him do it.
Long and Low - his neck is stretched out forward and down and you have minimal contact with the bit on the horse's mouth. Ask a Western rider - we English riders do it differently for most of the time. Get someone to show you the differences.
If you know the meaning of "weak top line" then get to know the meaning of "long and low" and "collected"
PS What do you mean by "pointy"?
Very well put!
Hill work is your very best friend for building all muscle - not just the topline. I've found topline is one of the last things to be truly built up, everything else has to come first and it can all be done with hill-work.
You do need to give your horse breaks from it though - you can't ride hills absolutely every ride, but you can put a little bit (a couple times up and down) into each ride, and some rides you can concentrate more on it. Remembering though that work on the flat, and especially work on hills is MUCH more tiring than jumping. Your horse will tell you how much is enough and if you push, like Barry said, how much is too much. At first, if your horse is as underdeveloped as you say, even 15 minutes of hill work could be enough for one day.
I've found that the slower the better with hill work for building muscle. At first - until you see good improvement in his fitness and in muscle, just walk. Get a nice steady walk in the up-hills, you want him to push off nicely to get some momentum, and a nice SLOW walk on the down-hills, really feel for his hind legs reaching underneath him.
When you see a noticeable difference to his fitness (he isn't puffing after 10 minutes) and in his muscle (he's physically starting to show muscle in his quarters and his back), then you can add in bouts of trot (not a lot!). Trot the uphill (or part of it, if it's a long hill) and walk the down hill (again feeling for him stepping beneath himself and using his hind end). Don't worry about what the front end of him is doing at all during hill work, you want to feel the hips. If he is working straight through the hips and straight beneath you to the shoulders then he is working evenly on both sides. Vary the pitch of the hills too - don't always do steep ones, shallow ones can work just as well.
You don't need to canter hills at all to get the benefit of them. Cantering hills I've found is good later on in his training to help with pushing evenly from the hind end (as pretty much every horse has a dominant hind leg). Even then, I only canter up, feeling for if he's square behind (not tilting one hip more than the other) and I walk down, looking for him to stretch beneath himself.
Hope that helps!
Pole work is also very beneficial to building muscle and balance. You can do pole work in "long and low" where you're asking your horse to stretch through the top line (this can be a good thing to do as warm up or cool down), and they can also be done while asking him to work up and use his hind end and bring his back up and collected.