Thumb or pacifier
Finger sucking is natural. Some babies suck their thumbs even before they are born. Most children will stop sucking their thumbs by themselves by the time they are five years old if no one draws too much attention to it.
Pacifiers can also calm babies. Babies may prefer a pacifier with a nipple just like a bottle. One-piece pacifiers are safer. Never tie a pacifier around baby's neck.
Don't overuse the pacifier. Try other ways to comfort him first, like holding, rocking, or singing.