Both of what the 2 previous posters said is correct.
Leos will slow down eating, and force feeding is likely not a solution you want to enact.
Also, and most importantly, if your "leopard gecko isn't eating, but his faeces appears normal and he isn't having diarrhea" then, I assure you, he is eating something. When they don't eat, they have nothing to poo. Makes sense, right?
They don't eat much in the winter, so you might not notice if he's only eating a few items every couple of days.
If he's pooing, he's eating ... something.
Joel
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2.3 Leopard Geckos
1.1 Ball Pythons
0.1 Okeetee Corn Snake
1.0 Blair's Phase Gray Banded Kingsnake