His weight has definitely improved from what I can see, and I like that his new summer coat seems to have a little more shine to it.
He does look under-muscled yet, not only in his bum but across his topline and neck. It's hard to tell, but if he had his head up in the second side-on shot (most recent), it looks like his withers would still drop off sharply to his neck. Of course, it looks like he has a lot of withers anyway.
The best way to build muscle is to get him walking. Trail rides over varying terrain are excellent - hill work to get him driving from behind and get his bum working. To build up his topline more, the best thing is to get him working from behind and lifting his back. I like ground poles (and later, cavaletti) for this, alongside tweaking your riding to emphasize forward from seat into legs into hands. Kind of a dressage-y concept, but perfectly applicable to any discipline. As he starts using himself over his bum and back, you'll see those muscles fill out more. Do a little research on long and low exercises - done correctly this is wonderful for longitudinal stretching, relaxing, and that bum-forward drive while rounding the back.
Since you're coming out of winter, I'll assume that he's rather out of condition anyway. Here's an article on general conditioning and bringing a horse back to form after a winter of perhaps minimized work. Understanding Conditioning as You Prepare to Use Your Horse More Actively
The first bit is very basic horse care, but midway through touches on physical conditioning and improving horses' fitness level.
Great job so far, and good luck!