I had the some problem on my truck a while back. For a temporary fix you can use an epoxy that you can get at autozone or an auto parts place. just drain your radiator, dry the surface area, scuff the area up with some sand paper, then spread some of the epoxy mix on it. Note that when you get the epoxy it has to be mixed seperate. It comes with epoxy and hardener. Also I would let it set for at least 24 hrs, or recomended on the package to get best results.
Good Luck!!