The wikia is your friend: http://fallout.wikia.com/wiki/For_Auld_Lang_Syne
"Arcade will then tell the player that there's something he has to do, and he will give the player all the gear he is carrying for them and run off. A short time later, he will come back with the Gannon family Tesla armor. If he plans to fight at Hoover Dam he will be wearing it and tell the player that he can't turn his back on his family and his past. If he plans to go back to the Followers he will give the player the armor and tell the player to use it well during the fight.
Either way, he will then go away and will be non-recruitable for the rest of the game. If he chose to remain with the followers he will go away (not respawn) and if the player follows him, eventually end up in the Lucky 38 suite with no possible option of recruiting him again. If he chose to fight, he will be at the dam along with the rest of the remnants, fighting for the NCR."