S.T.
if he's been doing it his whole life, it's probably not connected to the divorce. if it's worse, or he's exhibiting other anxiety behaviors, i'd focus on helping him cope with the anxiety, not on breaking 'bad' habits.
because unless his teeth are shifting, what's the harm? he's not doing it in school or in public. it's just a sleep thing.
you could try painting on the nasty-tasting stuff, i guess.
but i think you should be glad he knows how to soothe himself to sleep and leave him alone.
khairete
S.