Your cat is so happy and thrilled when you pet him he just can't control it. Its actually quite a compliment. I have one that does it as well. I have gotten used to it and how to pet to minimize slobber on my hands.
I think it's cute. It just means your kitty loves attention. My Isaac will do what i call fake suckling. He pushes his face right against my arm & kneads my arm like it's his momma kitty, but he doesn't actually suckle on my arm, but he dose drool. Sometimes I can feel it just running down my arm. I know it's strange, but I love it because it means he thinks I'm his momma!
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.
Years ago I lived next door to a family that had the sweetest cat. He used to come over to my house for visits and treats all the time and make himself royally at home. He was a real drooler too, especially when he was brushed or rubbed around the face and neck areas.
My Chynna drools a bit too when her and I are having a good cuddle session. Again it mostly happens when I'm scratching her chin and cheeks and sometimes her ears.
My Abby only does it when I'm brushing her cheeks, which she just absolutely loves! She's pretty quick though and smacks her lips and swallows before it seeps out of her mouth.