$4,200 for an 8 year old Honda with only 113k miles is an excellent deal. Your son could drive that car to 250,000 miles and it's still cheap enough to not bother with collision insurance. So what if the engine block actually does crack...just throw a junkyard or rebuilt motor in for a couple grand and keep trucking.
Even better if it's got a manual tranny.
I mean come on, it's a $4,200 sedan. It's not like this is a $35,000 truck. I wouldn't worry about it. Kudos to your son for buying a car in cash in his price range. I know many fools in his position who would buy a loaded $50,000 truck on a 6 year note.