My cat isn't gaining weight?

Several months ago, I adopted a kitty from my neighborhood. He's acclimated well to the house and my other cats.
He was very thin, skin and bones, when I brought him inside, and he gained around two pounds.
Recently I got an appointment for him to be neutered, but since it's been around five months, I'm worried because he still looks very skinny.
All my cats were flea treated and wormed for tape and round worms last week, but he hasn't gained any weight.
Currently he weighs very little and has very thin and sparse hair on the latter half of his back. Does anyone have any idea why he's still so thin?

That's a very difficult question to answer, as it could be so many things. The best thing to do is to get him a check-up at the vet and have them run some blood work. They'll look for things such as infection and thyroid issues that could be preventing him from putting on weight.

How are his eating habits? Is he eating enough? If he's not eating very much, then a loss of appetite is another concern of its own. And, again, a vet visit is warranted.

Right, I forgot to mention that he has a broken tooth. He will be going to the vet soon to have it extracted. It's a front tooth, broken down to the root. This doesn't seem prevent him from eating though.
I guess it's best to just ask when I take him in.

I would definitely get him to the vet.  Something is definitely going on.  How old is your kitty?  There are a lot of different things that can cause a cat not to gain weight. 

He's probably around 1-2 years. He's got lots of energy, and he seems to get along reasonably well with the other cats.
I just bought feliway today, because I have six cats total, and he always tries to climb the blinds.