Can an indoor cat ever be an outdoor cat?

I would say it would be okay, but perhaps you should start him on your porch or backyard, so he can get used to it. You have to get him used to the park gradually. If he hasn't seen grass, he would not know what it is, let alone a whole lot of trees, bushes, and flowers. You also have to make sure the park allows domesticated animals. Some parks don't, so you'll need a cat leash.