This question has come up before. You may wish to contact the Canadian consulate or law enforcement in Canada.
The response above is not intended as legal advice since it’s impracticable to provide thorough, accurate advice based upon the query without additional details. It is highly recommended that one should seek advice from a criminal defense attorney licensed in your jurisdiction by setting up a confidential meeting. Moreover, this response does not constitute the creation of an attorney-client relationship since this message is not a confidential communication because it was posted on a public website, thereby publicly disclosing the information, which is another reason to setup a confidential meeting with an attorney.
Not if it's in the last 10 years. If so there are various work arounds including a certificate of rehabilitation and a temporary residency permit. You will need the help of an immigration lawyer with DUI experience to get these, although the forms are available from any Canadian embassy or consulate.
Canada considers all DUIs to be felonies even a Misdemeanor DUI in the United States. Normally, Canada will not admit someone with this conviction directly. However, there are attorneys that specialize in Canadian Immigration, and they can get you a special Visa so that your son can be admitted into Canada. So, the bottom line is yes your son can travel to Canada on a Misdemeanor DUI conviction, but you need special assistance from a Canadian Immigration Attorney.