I still have 1 more milk tooth now. Last month i took out the other one. My milk tooth didn't drop because i didn't have adult tooth below that milk tooth. Dentist said it could happen, but rarely ( not to have some adult tooth below the milk tooth ). Maybe you should bring your son to the dentist & ask them about it & if he also don't have adult tooth below the milk tooth, it's best to take extra care of them, make them last as long as they can. If not if the tooth drops, it will affect their placing as there's a gap there. Teeth are line in a row, pushing against each other.
So if it has a gap that is not gonna be fill up, it will make all the other teeth move out of place. & one fake tooth would cost $4000-$5000, medisave claimable $1200 only. Others is cash. It's painful. So my advise is to check properly with the dentist & if really no adult teeth below that's why his teeth not dropping, best to keep the milk teeth healthy & clean as long as he can.