31/2 month old - finger sucking?

I would definitely get him to stop. I sucked my thumb when I was little, and it wasn't stopped by my parents. I sucked my thumb until I was about 20 years old, and believe me it is quite embarring when you go away to college or have sleepovers with friends. It also completely screwed up my teeth, and I have had to have all kinds of orthodontics and braces to fix them, none of which I would have needed otherwise. Speak to your dentist, and if needed and orthodontist that your dentist recommends. They are chock full of ideas on how to stop finger/thumb sucking. If something doesn't work, they always have other ideas.