I am not a big believer in stopping thumb sucking. It is a self soothing mechanism. If the child is stressed, he needs it and will give it up when he is no longer stressed or finds other outlets.
Sometimes, it is just boredom, if so, try and get him interested in something else. Ask him to wash his hands after any activity like petting the animal as he puts his hands in his mouth. And before touching anything you don't want his saliva on. this raises his consciousness. You can also associate it with going to bed. "You are sucking your thumb, you must be tired, go take a rest in your room." This will raise his consciousness too.
My then 4 yr old stopped when the dentist spoke to her, her sister stopped when my inlaws made fun of her and then went back to her bankie and thumb sucking until she was 10 or so. It ruined her relationship with them.
She has great teeth by the way. Each kid is different.