About a week ago I started the Dead Money DLC and as you all know you are not allowed to take followers or companions so I had to make my follower, Boone, leave my party. Almost immediately after I made him leave my party it said that he returned to Novac. Now that I am done with Dead Money, whenever
I go to Novac to look for him I cant find him. Where is he?!??! I have looked in the dinosaur's mouth and that other dude is there. Please help me, I really need his sniping skillz.

Check the Lucky 38?

Wait until nightfall. Boone is the night guard, and the other guy you're seeing is Manny Vargas. Just time-skip until around 9:30 PM and you'll see Boone pop up in the dino's mouth. Happy trails!

Or you can go to Boone's room in the motel.

Check the dinosaur, his hotel room, the Lucky 38.
Additionally, if all else fails, go to Vault 22 and take elevator, that will respawn him next to you, as long as you didn't fire him for his services the last time you parted.

does that work on all the companions or just boone?

I would look at the Lucky 38, his room in Novac, and the dinosaur's mouth @ night. If all else fails, idk what to tell ya.

From the sounds of it have you checked at a later hour? Looks like you are at the dinosaur's mouth during the Day. Manny is there instead of Boone.

Boone is only in the dinosaur's mouth during the night hours.