Well, if it is a juvi male, there is little reason for him to suddenly stop eating unless there is a problem.
A few quick questions:
Substrate - Is it tile or papertowels?
Is the enclosure unchanged since he stopped, no moving things around or changing the location or traffic around it?
UTH for belly heat?
If you answered No to one of these questions, then that could be the reason. Impaction in the first (vet ASAP) or stress in the second (let him adapt or return to a quiet spot) difficulty digesting in the third.
At this point, I would really highly suggest a vet visit because a juvi that suddenly stops eating will not be fixed by ensure. We can usually get in the day we call or the next so he shouldn't have to lose much more weight.
Ensure is best for supplemental use when all other conditions have been ruled out including parasites and infections. Ensure can also be used to help produce a stool sample for the vet if parasites are highly suspected. In this case, I would suggest not assuming anything and take the whole gecko in. They can swab his rectum and get a smear for parasites and also check his mouth, eyes, and nose for signs of infection. If he gets the all clear, they usually have a milk formula with cricket puree which is even better than ensure to try feeding.
Good luck, please give us an update on the little guy!
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0.1 Albino Leo Gecko
0.1 Crested Gecko
1.0 Dear Boyfriend
Departed: Harvey and Spock