ok, first, get it off the sand and put it on papertowel or carpet or tile, if he licks up the sand he wont beable to pass it and he will die
Secon, get that gecko to a vet, i dont want to alarm you, but he should be alot fatter with a full tail, the only leos i've seen that looked like yours had parasites that need tpo be treated or else they're fatal, you got him from a pet store correct?, take a fecal sample to a vet and ask for a fecal exam, a vet should be able to determine whether or not there are parasites and he'll treat them
