My Cat Has White Flakes On His Skin But No Visible Fleas. What Can I Do And What Might It Be?

Its a condition just like regular dandruff on us humans, and may be a minor allergic reaction to something new in the home. If it doesn't seem to be causing the cat any discomfort, it does not necessarily need to be treated, and may clear up on its own. Failing this try to find a mostiorizing lotion, or a food with oils to help the dry skin.
Failing this, see your vet.

Dry skin give him a nice bath, and it should clear up..