I'm wondering if it's normal. Do leopard geckos eat less and sleep more in the fall/winter? We saw this happen last year too. Still I want to be sure all is well. He seems lethargic.
Mine tend to get a bit sleepier and some go off food. I weigh mine every 2-4 weeks or so to make sure they're not losing much weight. Up to 5-10% is normal. More than 10% and I would start to get worried.
You get a gram scale (Staples is a good bet) that weighs in grams or ounces and either put the gecko right on the scale or put it in a deli cup (obviously you have to either subtract the weight of the deli cup or use the "tare" function to zero out the deli cup weight).
My geckos eat a lot less in the fall and winter. Most lose very little weight, but I have one male who loses about 20 grams every winter. I have checked the temperatures, taken him to the vet, done assisted feeding , but this is what he does. Right now he's at 90 grams so he can afford the loss.