Hello. Well, kids are still very orally involved at that age even though it is less socially acceptable for them to be that way. You will often see kids chewing on sleeves, back pack straps, the collar of their shirt, etc. It's important to know that usually they are self soothing. So, when they have their finger in their mouth and they are playing around with the saliva from that, you switch to something more acceptable. At home, give him or her a sugar free piece of gum or candy. Licorice is fantastic as well. They make chewing items for kids--- a hard plastic P or T that is either smooth or textured, caps that go on end of pencils made to chew on, chewing wrist bands, etc And good discussion about what is okay to do and what isn't. Spit is like snot . . . icky and full of germs. Kids won't want to have his spit on them. Talk to him about that. Give him alternatives and then cues to help him remember. The cues shouldn't be such that they embarrass him but jut remind him to switch to the 'other' thing you've told him is okay to soothe with.
But understand that he sis probably doing this out of impulse to soothe. Adults orally soothe too . . . that's your smokers and gum chewers and nail biters. So, offer alternatives understanding the impulse. good luck