My fathers airsoft gun is always firing full auto?

sounds like the trigger may be stuck, or it's somehow making a complete circuit. If it's an ARES, then that would be the clicker got stuck down, but on most AEGs it would be the circut switch

Sounds like something in the gearbox got jammed probably something to do with the trigger. I have heard of this happening with JG guns. It just jams then they move the fire selector switch around and it stops firing full auto all the time.

might be a bad selector switch, you should think about adjusted the selector switch. here is a video http://www.airsplat.com/airsoft-gun-troubleshooting.htm that might assist you. should be the fourth one. i would not subject disassembling the gearbox though, it will be very difficult to put back together unless you have some real experience doing it. you might want to send it back for a repair or something.

I think the more important question is why a grown man is playing with toys :/