BaseballMom2020: I haven't bought helmets in years, but based on the link I posted above in this thread, most should carry a label indicating how long they remain 'certified' as meeting NOCSAE standards. Four years seems like too long to me; but you could always stop by a local store and see how long the new helmets there are labeled as lasting--that would give you a sense of whether yours has 'expired.'
I'm very concerned about this issue, but I wouldn't pay $500 for the helmet advertised at the link you gave. I assume those are old Rawlings S100 models that someone had in a warehouse. They may be in better shape than they would be if they had been banging around in a gear bag for years, but they are going to be fairly old.
I think the S100 is still available to MLB teams (?). Rawlings probably stopped selling it to others because many, like your son, thought that helmet was uncomfortable. You might try writing to Rawlings to ask if you could buy an S100, although I expect it would take a while to get a response.
Easy for me to say since my kid is a PO now and no longer hits, but I wouldn't get overly concerned about the Rawlings mph ratings. Yes, the S100 should offer more protection than lower-rated Rawlings helmets; but their mph rating isn't based on any industry standard. An S100 isn't necessarily better than the top of the line model from a different manufacturer, even if that helmet doesn't carry a label saying it's rated to 100 mph. (Although I agree that the S100 seems to be at least as good as anything else that was, or is, out there.)
It would be great for players and parents if an independent standards body gave more comprehensive ratings. For now, NOCSAE seem to be the only folks doing anything in this area. I'd go with a well-known manufacturer and make sure my kid has a helmet that actually fits (how often do you see players wearing helmets tilted back so that their foreheads are exposed, or see helmet fly off when someone is running the bases?). Not sure you can do more than that.