My Ds1 kept his until 4y & then gave it up on his own when he gave up naps, my DS2 sucked on the arms of a bear until he was 4.5yo & then gave that up without my pushing it... both didn't have any long term effects on teeth as DS2's open bite caused by the bear closed up to give him a "textbook" (according to the dentist) bite.
I think if it isn't causing you any issues & he sleeps well with it then there is no reason to get him to give it up if he only has it for sleep.
One thing a friend of mine did was start at about 3yo "4yo's don't have a dummy, it is ok for 3yo's but not for 4yo" & about a month before her DD turned 4 she said "I'm ready to be 4, here are my Dummies, can I have my Birthday tomorrow, I don't want to wait"... so they had it early, it was very cute, hearing her say "my proper 4yo Birthday isn't yet, but because I gave up my Dummy I get to be 4 early"