You might want to try spraying "Bitter Apple" on the area where he likes to lick. They hate the taste. It's sold in any pets store and has a green and yellow label. Also, cats hate citrus. That's another possibility.
We have a deep-set window where the cats like to sit. This is an older house so apparently there is a leak around the window and the plaster is coming off in chunks in this area. I am constantly finding our cats EATING the plaster! Any clue as the why they'd want to nosh on plaster?
It never occured to me to spray biter apple there *slaps forehead*
Sometimes these odd cravings cant point to a vitamin deficiency. In either case, I would first try and suppliment thier diet with a multi vitamin for cats or the 3v caps which are an omega3 fatty acid along with vitamins A,D and E.