The best way is with a thread insert.

 

Helicoil by brand name

 

This is the insert I was talking about, not helicoils, though they will work but not as well.

 

This is the insert I was talking about, not helicoils, though they will work but not as well.
View attachment 329564

Those look like TimeSerts. I've heard they are better than HeliCoils, but much more expensive.

 

would you know what size insert that would take?

 

The oil drain bolt is 14x1.5mm.

 

Thank you very much I'll let him know it's much appreciated.

 

Another option is an oversized drain plug.

 

In an emergency, I've used a bottle stopper, the kind that has the little lever you press down which causes the rubber to expand into the hole. It's not really under any pressure but just don't use it as a permanent fix.