1988 Chevy Silverado manifold bolts

I have a 1988 silverado 454 engine that needed manifold gaskets. My brother in law tried to install them, but on the right side on the bottom in the back, he tried to screw the bolt in and stripped the threads inside the motor. He also put some type of putty or something on the bolt(which hardened inside the whole where the screw is supposed to go) to try to get it to stay in the motor and tighten up. Now everytime I try to put a bolt in, it goes in so far then pops right back out. What can I do to fix this myself? PLEASE HELP