As stated, a DUI is considered a felony in Canada regardless whether or not it was a misdemeanor or felony in the USA.
Canadians allow Americans with DUI convictions to enter their country IF they have a 'waiver' issued by Canadian authorities. There is a substantial fee involved to acquire this waiver and not every first time applicant of this waiver is granted one. The fee is for each waiver application and each waiver process can be time consuming, meaning several months to acquire.
If you do not have enough time before you leave on this trip for your son to complete this wavier process, you have a couple options. Fly son to Anchorage or Fairbanks and pick him up there with plans to return flight him back to the USA. Or leave your son out of this trip.
Good luck and enjoy your adventure.
Lakeside