He needs to have a legal status in Canada when you send your application to sponsor to qualify for the OWP program. So long as he has a visitor record, that is current and up to date at that time, he would be in status. If, for any reason you both decide that you don't want to fill that paperwork out and he allows his visitor status to lapse, he is no longer in status and would no longer qualify for the OWP program upon filing an Inland application.
Depending on how things go working into the future, since you are still more than a full year away from actually applying, consider doing an outland application for him. He would not qualify for the OWP in that case, but it would certainly be a quicker process for both you and he. Currently it takes somewhere between 2 and 3 FULL years to get a final decision for the Inland route. Currently for a US citizen using the Outland route, it takes roughly 7 months to get a final decision. Of course, the choice is up to you both to make later.