A skinnier tail means that he is not eating enough so that his body is taking the fat stored from his tail. Not a good sign. His dull colors could be that he is about to shed soon. Before a shed he wont eat.
How old is he?
Pictures could help as well if you can
Is your tank in a room with sunlight and how do you heat it? A Light? An under tank heat pad? (UTH)
A few setup questions:
1) How are you monitoring the temps? Analog or Digital? (I recommend digital as analog can be up to 10 degrees off or more.
Also Leopard Geckos need belly heat not air which is what analogs take the temp of (the air)
2)What are the temps of the hot/cold side of the tank? The hot side needs to be roughly 90-95 and the cool side 80s/70s
3) Do you change the water daily to avoid bacteria build up?
4) Do you have a pure calcium dish? Do you dust with Calcium, Calcium with D3 2x a week and also with the Mulitvit 2x as well a week?
5) Do you gut-load your gecko? How nutritious your crickets/worms are is how healthy your gecko is.
6) What do you feed your gecko? Crickets, mealworms, superworms, silkworms, waxworms, butterworms, or Dubia ****roaches?
7) How often do you fed your gecko and how long is the food left in there. Ie mealworms in a bowl everyday. Crickets left in over night. Crickets in a bowl with hind legs at knee amputated so they cant get out.
Hope that wasnt too much. I think all of that will help everyone help you help your gecko the best!
Edit:Note: Leopard geckos store fat in their tails so a healthy leo will have a fat tail. They store fat in their tails in case they cant get to a food source.
I hope you dont mind all the questions but I feel those are require to help you help your gecko out!
***
Trish