My cat is straining in the litter box and little comes out.

Go to a vet now. A cat, especially a male, that strains to pee and passes little or nothing may have a blocked urethra. This is a life-threatening emergency: waste builds up in the blood and can stop the heart within a day or two.

It is easily mistaken for constipation. Don't wait to see if it passes.

The long version: Where the litter box should never go

General guidance, not a substitute for your vet. If your cat seems unwell, call them.