you can't have a single nnid on two 3ds'

 

You can perform a system transfer to his 3DS, then any legitimately purchased content will be available for him to redownload (or should just appear on his home menu if you do a full sd card transfer). Non purchased stuff will not go with the transfer unless it's bring moved to an emunand not a virgin console and it's a whole sd card transfer. Also you'll have to have the latest system update on both consoles to do the transfer (emunand or sysnand) and you'll lose access to all your stuff on your console (which will behave like a newly purchased console)