Given that his family last heard from him in 1973, I think we can assume he died not too long after they had their last contact. His remains were found in 1980, off trail in Algonquin Park with no signs of foul play.
Algonquin is a huge park, nearly two million acres. In October it is stunning and there would be other people in the park for the fall colours. but depending on where and when he went into the park, he could be completely alone. Even in the height of summer, it is possible to be in the park and not see a soul for days. The trails start to close down as the weather gets colder. Back then, in the off season, a person could stay in the park for free, so it might have looked like a good deal for a guy without a steady income.
It took several years for his remains to be found, so I suspect he went off trail to find a cozy place to set up camp, but a person really can't survive in the park without a safe place to stay warm, supplies and a lot of knowledge. He might not have been able to find his way back out.