Good query and the first tip is here. If they are still sucking their thumbs, do not scream, nag or get angry with the boy. This would only intensify their sucking habit of the thumb and make it much more difficult to get them to stop. You can learn to stop finger sucking and thumb sucking online.

(Age 2-7) STOP Thumb Sucking - stop FINGER Sucking - The Hand Stopper thumb guard

There are some ways to get the toddler to stop sucking his or her thumb:

1) Taste – There are things you can put that taste pretty nasty on the thumb of your kid. These, of course, are absolutely secure and approved by the doctor. This enhances the poor taste of the thumb with an immediate thumb sucking stimulus.

2) Praise – For specific periods of time, if the child does not suck their thumb, make sure to praise the child. It provides optimistic affirmation of something well done.

3) My Special Shirt – This is a specially formulated program of shirt and behavioral pattern to help the child and parent conquer thumb sucking effectively.

4) Pay More Attention to the Infant – Due to a sense of vulnerability and the need for warmth, thumb sucking also continues.

5) Older Kids – Have them help pick the technique that they want to try. This will allow them more power over the stopping process.