All I can tell you on mine is that the old radiator I removed (original at 195K miles) had a blue plug. The plug itself is removable and replaceable. I looked at realoem and see some other little pieces there, but I don't think that's the fitting...I'm thinking that that's just 'pieces' of the plug...but I guess I'm wrong...or could be with yours.
First step, I'd say, is just replace the plug...assuming you did try to just tighten it. It's plastic, so overtight would prolly break it.
Also, sorry to say this, but when ET fails, it drips down to the bottom of ET and might look like it's just the plug...but it's not. So, just be careful about assuming it's one thing coz of where the drip is...water, you know, will migrate.
Tell you son to remember that if the needle starts moving rt. of center, car off and call Dad!
Had you been doing anything in that neighborhood? Is your son a fanatic who might've been reading around here and fiddling around with things?
Anywhoo...hope this helps. Make sure you've got the source of leak right is most important thing.