you havnt said how long you have had him, but you are doing the right thing by taking him to training classes, and sooner or later he will learn the basic recall and to come back to you, but you need to practice this on the lead first. As for taking him out over the park, I would use a flexi lead or tracking lead until you are sure he will come back. I rarely let mine off the lead to be honest, only when I know there are no other dogs around as they are easily spooked which is a different scenario to you. They have their free running exercise in the garden, and plenty of long walks on the lead. Good luck with him, you will need plenty of patience and perseverance.