my step daughter sucked her thumb until age 11.
pestering/punishing made it worse.
best to ignore it.
she had to decide for herslef that she wanted to stop. her teeth are ok now
give him something to hold/do to try and get him to have to use his hands for other things.
if its just at sleep time, not much you can do - but thats better than all day long.
my dsd wore gloves to bed at one stage