Some issues with installing a different radio/receiver..
 

Trying to install a used 2.4ghz controller and receiver into my sons stock 2wd slash. It seems like there is some kind of interference. The steering servo will randomly turn on its own, the throttle will randomly hit the brakes for a split second while driving. Sometimes it all works just fine for a couple minutes? I originally thought I was just getting some interference from my kids wireless xbox controller so I had him turn it off, but that didnt change anything. Any ideas I can try?

Exactly what kind (brand, model) of radio are you using?

I hope you didn't destroy the antenna wire by pinching it in the antenna tube holder. The Rx box has a hole where the antenna wire goes through the bottom and then up through the center of that base where the tube goes. Then you should have a blue aluminum nut that locks the tube in place.

It's an Exceed RC brand? I took it out of one of his buggies that he doesnt use anymore.
I took the stock Traxxas 2.4ghz receiver out of my truck and installed it into his and it all worked great. I guess its just that Exceed receiver?
I will look over the antenna and see if its damaged.

Exceed=Hobby Partz=Fly Sky

If you can find some sort of identifying part# on the receiver, you can probably replace it with one from that store/brand for about ten bucks. However, those systems are known for lag and receiver failures so the Traxxas system is probably a better way to go anyway.

If he did manage to mangle or pinch the antenna, is there a fix? Or would he need a new rx?

contact traxxas and get a new antena wire. its a very simple fix even for newbs.