Make sure your insurance will cover third parties across the border. Although in Ontario you can safely loan your vehicle to anyone and they have coverage, I could be wrong, but I want to say that I saw something that did not extend that same coverage across the border..that it may be named policy holder only.
The LAST thing you want to have happen is have him get into an accident in the USA and find out your coverage doesn't cover it, especially with how litigious they are down there - even a fender bender in a parking lot can magically turn into someone suffering "debilitating life altering injuries", especially if they (or their lawyer) gets wind of the fact that it wasn't the registered owner driving it, that the driver isn't even from Canada Or the USA, that the car is from Canada, that they were driving on an international licence, etc etc etc.