Please give me tips to stop my baby's thumb sucking habit...

Place lemon juice on his thumb so it feels bitter when he sucks on it haha. On a more serious note, if you'd like to find an alternative, you could try a pacifier? If you'd like to stop the habit permanently, you could try getting some tips from here: http://www.webmd.com/parenting/features/9-ways-to-wean-a-child-off-thumb-sucking