You can move non-mayor player characters from one town to another. So, he could be the mayor of his town or a side character in his town and visit you, or he could have a resident side character in your town that he would have to play as on your copy of the game.
You have one town for each copy of the game. You have a copy, he has a copy. Two towns. The first human character you make is the mayor. Any human character can visit other towns. The mayor cannot move their house to another town, and if you delete your mayor, it deletes the town. You can make up to three other human characters after the mayor. These characters can be deleted without consequence, and you can also move them to a town on a different game cartridge.
I think what you are asking is if he moves a character to your town, can he, with HIS game, boot up and play the character in YOUR town? The answer is no.
But you can have multiple characters in your town, and he can play as a person who lives in your town if he is playing your copy of the game.
Basically if you want to live together in a town, you have to share one game. If you want to play together at the same time, you need two separate copies of the game with two separate towns. You can't really have both of these things at the same time.
