The letters from the first board came about 5 days after the status changed.
Now that is a new one, the ROO called you and said a letter was coming but the status has not changed. If the letter is one informing you that your son received an offer without the status page changing, that would be a new one. This year has been so different compared to others that it would not be a surprise. If this ends up being the case there may be some here who did not see a status change but may still get good news.
I would call the ROO back and see if the letter is indeed an offer, I'm not sure I could stand the wait.
Good luck, I hope it's good news for your son.