Problems with my young leopard gecko

Question
I recently bought a leopard gecko about 2 months back or so and I would guess that his age is a little older than that. When I first got him he was energetic, ate, and pooped regually. Now though, for the past week or so he hasn't been eating quite as much and is acting very lethargic. He spends most of his time in his hide on the warm side of his tank. I can pet him and he will open his eyes, but he will close them again rather quickly. I can place food right and front of him and he won't touch it.(crickets and mealworms) He has not been gaining weight either and his tail is rather skinny and hence so I fed him a few waxworms(not at one time though, over the course of a few days) He went after those at first but now won't touch those either. But even now I can pick him up and hold him, but he will just stay still and it almost looks like he is asleep in my hand. When I put him back in his tank he basically just stays in the spot where I put him, lays down, closes his eyes, and goes to sleep again.
Tank info-
He is in a 10 gallon tank with both a warm and cold side. I have an under the tank heater for the warm side and have a day light on him durning the day and a night lamp for him as well. I switch him between reptile carpet and paper towls. He has a calcium dish in his tank as well as a shallow water dish. I dust his crickets daily. He has two hides on each side of the tank and I also place a moist hide in the tank when I see that he is getting ready to shed.

Please tell me anything that I can do to make the situation better. This is my first reptile pet so I'm not as experienced but there must be something I can do. Thank you, and I appreciate any help.Please hurry with any responses though!

Answer
Hello Lauren,

What type of substrate do you use, sand, tiles, or paper towels?
He could have parasites which are causing him to lose weight like this.
Do you use a basking light on the end that you have the heater on?  Do you
leave the undertank heater on 24/7?
I would either try to get a fecal done on him, or go ahead & try to deworm
him with Panacur.  
You can get some chicken, turkey or beef baby food to feed to him through a
syringe since he is not eating very well right now.  Just use either a plastic
syringe or a plastic dropper to feed him by dropping the food onto his nose to
see if he will lick the food off that way.
Do you have any pictures of him for me?

I highly recommend getting the baby food into him right now so he can get his
immune system boosted.  Don't forget to include his calcium & vitamins in with
his food for him.  
Has he been going to the bathroom lately?
Let me know how he is doing.

Tracie