There are several reasons your gecko could be squinting:
– loose substrate (like sand, gravel, lizard litter, etc)
– nutritional deficiency
– infection
– injury
– retained eyecaps (bad shedding)
Get rid of your current substrate right away (I know pet stores recommend it but they are wrong). The most common cause of eye issues in geckos is loose substrate. A bit of sand in the eye can cause very bad infections ..which can even cause blindness and/or the need for surgical removal of the eye. Loose substrate can also cause respiratory problems and impaction/compaction (from the gecko ingesting the loose substrate). The best substrate are the solid types like: reptile carpet, non-stick shelf paper/liner, paper towel, plain newspaper and ceramic tiles.
If Charlie is acting normal, other than the squinting, you can gently clean his eyelids with a lukewarm cloth. Also, try a few drops of preservative-free saline solution (like people use for their contact lens) twice a day. Do this for about a week. If he does not get better, or gets worse, take him to a reptile vet.
If Charlie is not acting normal (not eating, lethargic, etc), he should be taken to a reptile vet. He may need antibiotics for an infection, eyecap removal ..or treatment for an injury.