well, first get your son to invite you, in the mode you want to play (in this exemple versus), so ask him to go to versus, then choose a character, then press R2. choose invite, then ask him to send you an invitation.
then on your side, you have to wait on the main screen where you choose wich mode you want to play, then when the invitation pop on the screen, press X to accept it. unless you already press triangle when the invitation poped, then you will ignore invitation. simply restart the game, and try the invitation again.
hope this help.