Yes that gecko looks pretty thin, and it may be his enclosure setup?
You need many many more plants (I shoot for around 50% full of plants in my enclosures) and lots of branches/sticks/vines to climb on. Here's an example of a 10gal I had set up for one of my juvies:
I also would get rid of the reptile carpet (toenails and teeth are very prone to snagging in it and it's VERY hard to actually get clean- breeding ground for nasty bacteria and parasites) and I'd go with paper towel instead. You may switch to something like Eco Earth down the road if you want something more natural looking, but I would not at this point since we aren't yet sure what's going on with your gecko. Paper towels are easiest to keep clean and monitor for poop to be sure he's eating regularly.
Also, is that a heat lamp? You probably do not need a heat lamp. Room temperature (in the 70s F) is ideal for crested geckos, and I would not add any heat at all unless the tank drops under 60 F (some people go down to 55F, but I personally would add a bit of heat at that point).
I also would recommend picking up a digital hygrometer and thermometer, as they are much more reliable than dial types.
Last but not least, what are you feeding, how often, and how often are you misting his enclosure?
If he continues to lose weight, I'd encourage you to take him and a fresh fecal sample into a herp vet to check for parasites.
