You can't really get water 'stuck' in your ear for all that long. There's nothing to really hold it in. If he feels like he can't hear well out of that ear, most likely something is blocking it. That could be something in front of the drum like wax or a foreign body or somethng behind the ear like an infection or air trapped because of blocked tubes. The wax is most likely if this has been a long standing or recurrant problem. The foreign body is most likely if he is under the age of ten (sorry, I'm a pediatrician). The other things may be occuring if he has other cold symptoms or congestion. The other major possibility, especially if he banged his head into the water and is now feeling a sting when he puts drops in, is that his eardrum ruptured. I'd definitely stop using drops, and get someone to look in his ear. That will solve the mystery very quickly.
Rachel