B.H.
Maybe stop him before he does it and put your hand over his mouth and don't remove your hand until he is ready to not do it. Even if he spits in your hand it's gross but keep your hand on over his mouth for a good few minutes to annoy him enough to not want to do that anymore.
When my daughter use to get mad and call me bad names or be mouthy I took scotch tape and put a piece over her lips. It didn't hurt her and she was able to remove it immediately but she hated it and got the point.
Or you could give him a rag and insist he cleans up his spit and explain to him how that is disgusting and unacceptable.