My 17 year old son wants to travel with his 2 adult sisters to Canada for a 2 day trip but his passport has expired, and the trip is scheduled for April 24,2018, what does he need?
He needs a WHTI-compliant document. If he's traveling by air, that will likely be a valid passport. At the land border, it's possible he has a valid Enhanced Driver’s License (issued by a handful of border states) or Passport Card, and he could use that instead.
Fortunately, he still has time to get a renewed passport before his trip. Contact the National Passport Information Center online or by phone for an appointment at the nearest passport office. That page has links to the required form and documents he'll need. They can generally issue the passport the same or next day.
US CBP also recommends he carry a letter from both parents giving permission to travel internationally.