Sure hope it is good news - but I guess you ought not get your hopes up to high, yet. My D's did exactly as you described. She had gotten an LOA in October and she had passed DoDMRB. Her noms came the last week of January and they showed up on the web page. Then, after a week or two the page turned exactly as you indicated. Finally, a few days later the big envelope arrived. About a week after the big envelope came, the web again changed to read "appointment offered". It could be a coincidence. Good Luck - and maybe be check the mailbox for a neat grey envelope with bold print on the outside, but better safe than sorry - maybe don't tell your son. Wouldn't want to get his hopes up only to be dashed.
Please let me know - I sure hope it is. I would hate to have gotten your hopes up for naught.