What actually unlocked the Previous Assassin's outfits

Some of the outfits (namely the iconic outifts from AC 1-3) are unlocked if you have those games linked to your UPlay account. This might explain why you have them and your Dad doesn't.

However If you have any XBOX/PS3 saves from the previous AC games, the Altair/Ezio/Connor robes will be unlocked and ready to wear when Edward has access to his quarters on the Jackdaw after the Raise the Black Flag mission. The robes are equipped via UPLAY and cannot be worn offline.

So to answer your question more clearly, these outfits are unlocked on console by playing previous games. AC 4 will read you save games and unlock these outfits, but they cannot be worn/unlocked offline - which is why your dad does not have access to them.

EDIT: You can get these by linking these games or by linking 1 - 3 games in the series (i.e. Assassin's Creed Brotherhood and Revelations also count).