The character is adopted into a Wealthy family. If he is an orphan, it is a true adoption. If he has living parents, he is treated just like a son. (work with your DM to determine details)
Martial training: In adolescence you trained with the weapons of your people and learnt how to use them proficiently (work with your DM to determine details)
When you become an adult you learn to used a weapon (of your choice) appropriate for your culture and social standing. (work with your DM to determine details)