ALL!! Don't let your son make the mistake of relying on e-mails that could be delayed, misdirected, placed into a spam folder, etc. He should be checking each portal at least once per day. It's not that much work, and will thin out as IIs come in.
No school is going to advise you to rely exclusively on e-mail. It is a convenience (as is a phone call), but the official means of communication for everyone is the portal, since it has to be updated in order to allow interview scheduling.