It's going to suck but you can try hot sauce on their thumb, painting his thumbnail, tape a mitton over his hand. It's hard to break it (I used to suck my thumb) but there's a chance that he may just stop on his own. There's also a chance he won't stop until you fill his mouth with orthodontic equipment
Girl, good luck. My daughter sucks on her fingers at night. I'm hoping until habit will go away as she grows up