Ok, if his backbone and hips are still sticking out, then he's still pretty underweight. Have you either de-wormed him recently or had a fecal count done recently? He may be carrying a worm load from your description.
I'd read the instructions on the back or tag of the feed bag, that give you directions on how much to feed and figure out what he SHOULD weigh vs. what he DOES weigh (you can use a tape measure if you don't have a weight tape to figure that out) and then start feeding him up to where he'll gain the weight he needs.
Gigi is a horse I had sold and then took back because it was reported to me that she was being starved. This pic is approximately 1 month after I brought her back home, 12/3/11. I had taken her in to get her teeth floated and the vet weighed her at 810 lbs that day.
She SHOULD weigh around 1100 lbs so the difference is approx 300 lbs. I'm feeding Strategy GX, and the feeding instructions are to feed an Active Pleasure horse 1/3 to 1/2% of body weight per day in feed and to feed 1.2% of body weight in hay.
So.....to maintain her at 810 lbs (not what I want but for example purposes) I'd feed, 2 1/2 lbs to 4 lbs/day of Strategy and 10 lbs of hay.
To MAINTAIN her at 1100 lbs, I'd feed her 3.5 - 5.5 lbs/day of Strategy and about 14 lbs hay.
I started out feeding her 1lb Strategy 2X/day for 3 days, then I upped it to the same amount 3X/day, then went to 4X/day. After 2 weeks I had her on 2 lbs Strategy, 4X/day and after 3 weeks I added Amplify, a high fat supplement, to help her gain weight faster and safely.
Here's what she looked like on 12/30/11:
I'm giving her about a 3.5 BCS in this picture. I can see her withers a little to prominently, her hip bones still look like door knobs (they used to look like a coat rack) and if she moves right, you can still see her ribs a tiny bit. She's still got a ways to go, but she's gaining. I will be happy if I can keep her gaining all winter, and if she comes out in spring better than she is right now.
Sad thing is, when I bought this mare at an auction, she was thin. Not starved but thin. Here's what she looked like when I bought her:
And here's where she was when I sold her:
She was a little sucked up after a long trailer ride in this picture but I think you can see there's quite a difference.