"When John, who was in prison, heard about" or "When someone told John, who was in prison, about." Even though Matthew has not yet told the readers that King Herod put John the Baptist in jail, the original audience would have been familiar with the story and understood the implicit information here. Matthew will give more information later about John the Baptist, so it is probably best not to make it explicit here.
# he sent a message by his disciples
John the Baptist sent his own disciples with a message to Jesus.