Can he not apply for Canadian passport from abroad?You can leave with your current foreign passport just after the oath.....
.... but you won't be able to return to Canada.
Actually as soon as you are Canadian, you need a valid Canadian passport to re-enter Canada.
It seems that they are ways to get a travel document from abroad in case you don't have a passport. However I suggest that you stay a bit longer after the oath so you can get a passport before leaving.
All details: https://www.canada.ca/en/immigration-refugees-citizenship/services/visit-canada/dual-canadian-citizens-visit-canada.html