Your 14 year old nephew that is in visiting from Alberta cannot drive in Ontario with his out of province learner's license.

The Licensing Administration Office of the Ontario Ministry of Transportation (MTO) told us that if someone is a visitor to Ontario and wants to drive while here, they must be at least 16 years old and have a valid driver's license from their own province, state or country.

This division of the MTO also stated to us that they believe learner's permits, such as their own G1, are only meant for practice driving in Ontario, or issuing province, with a licensed accompanying driver present at all times. So even if your nephew was 16 it does not appear that he could drive in Ontario with just an Alberta learner's license since Ontario does not find other province's or state's learner licenses to be a valid driver's license.