He might not know a litter box, being an outside kitty.

I do with some adult cats what I do with kittens - put them in the box and take his front paws and make them "scratch" the litter a bit, as if they are covering up their stuff. Usually the light goes on.

He may want out - may have to go potty - or may just be meowing at something outside. I forgot - is he neutered?

hugs
Catty1