You don't really say much, other than the cat meows at night. Consider the following:
-Is the cat neutered? If not, he may be meowing to get outside for mating purposes. Get him neutered. It can take a couple weeks following neutering for the hormones to be out of the system.
-Is he hungry? Cats need to eat two times per day. Make a mix of 1 cup dry food/2 tablespoons canned/1 cup water. Give him a hearty helping just before bed and see if that helps.
-Who feeds the cat? If Mom feeds the cat, you should begin to feed him. That way, he will want to be with you instead of her. Feed him both morning and night.
-Where does he sleep? Allow him into your room if he is lonesome so he can sleep in there.
-Play with him toward evening so he is more worn out.
-Some cats like to be outside. If he is not afraid, allow him outside ONLY DURING THE DAYTIME. This may help if he is just bored. But, if a cat is outside at night, it can get seriously harmed or even attacked by nocturnal animals.
-Is the cat alone during the daytime? He may be bored and wants more attention. If that is not possible, would getting a second cat for a friend help him? Of course, introduce them SLOWLY, as in one cat kept in the bathroom for at least two weeks.
He may just want to sleep in the same room as you.