M.S.
Kids potty train through the night WAY after they potty train in the day. Girls will often make the change about 6 months after being dry all day, but boys are commonly in pull-ups at night until they are 4 or 5. There is nothing you can do to change whether or not he stays dry at night, so you should stop trying. When he is ready, he will just be dry every night. When he is dry for a week in a row, then you can try underwear at night again. He is not forgetting to come tell you, he is peeing in his sleep without ever waking up or knowing what he did. You have to be very careful not to make him feel guilty about something he has absolutely no control over.