He sucks his thumb

We have been dating for 2 months now and one night I slept over. While I was still awake he was in a deep sleep and I noticed he was sucking his thumb while asleep. Idk what to do about this. Should I tell him. It doesn't bother me bc its just probably a comfort mechanism. The inner wannabe psychologist in me wants to know more about why he does it and see if its an occurring thing. Another part of me just finds it adorable and weird. Should I tell him or just leave it alone.

Idk if it matters but we are both 19 turning 20 at the end of April.