Just put an old blanket or sweatshirt or something on your lap first. There's no way to stop it. And as the others have said - it just means he's blissing out.
And if you don't like the drool on your hand or arm or whatever - maybe try introducing a brush. It can take a while for cats to get used to what a brush is, but all of our kitties go insane for having their cheeks, chin and heads brushed. Between the brush and something on your lap, you can avoid the drool if it grosses you out.