My child sucked her thumb until she was 7, think she started before she was born...I see this as a similar problem. Start by talking to him and encouraging him to have the desire to stop the habit. Help him to see how others may see the problem...maybe video tape. Express true concern for him...you don't want him to hurt himself and talk about how when habits start they are hard to break and tell him you are afraid that this might be one that will be hard for him. We are Christians so we prayed about the thumb sucking, and asked that God would help her overcome it, and I asked her permission to bring it to her attention when she was doing it (I'd give her a "secret sign" to remind her that she'd stuck her thumb in her mouth.) Give him something that he can do while concentrating...maybe one of those squeezy things they give elderly folks to strengthen their hand muscles. If you see him biting his hand, hand him the soft toy. Good luck!