Some kids are very oral, so this can be completely normal. I understand how aggrevating this can be though especially when you're out in public and his hands are full of germs. When he's got his hands in his mouth try to distract him. Singing songs with hand motions is one excellent way because you don't need anything else so you can do it anywhere. Songs like Patty Cake, Itsy Bitsy Spider, and the wheels on the Bus. Although you can come up with hand motions for all nursery songs, etc. He might be hesitant at first but with a little practice and routine I bet you'll be able to get those hands out of his mouth. Also, building with big Legos or blocks are a great way to engage little ones with their hands. Spend some time helping him build by constantly handing him blocks to put into place. The trick is to get him out of the routine of putting his hands in his mouth. So the key is to engage him in an activity that requires him to use his hands. Play doh and finger paints are other great ways to keep his hands busy and it's non toxic if it does get in his mouth.