You may put on Sesami oil on his thumb and see if it works .. My little one stops after putting it , he dislikes the taste and stopped ...
if he's tired, introduce a new bedtime routine for example, extra story to help him wind down without his thumb
if he's bored, keep his hands bz with engaging acyivities like drawing, puzzles or play dough
My nephew put plaster and he felt weird putting his finger in his mouth and stopped ..
create a sticker chart with a sticker each day he doesnt suck his thumb
if he's anxious, offer comfort in other ways
start with shorter goals onto longer goals
praise privately as well as publicly