My dog will not "come" when called when he is outside but he knows what it means and always comes when we are inside.
The sights and sounds of the outdoors are a powerful enticement to even a well trained dog. There is no greater test of your training then for your dog to respond consistently even with exciting distractions all around. For your dog's own safety it may be better that he be under an invisible control until you are absolutely certain that he will be consistent off leash. To make that transition safer go to your local fishing store and purchase a roll of heavy test fishing line and a heavy swivel. Attach the swivel and line to his collar along with his regular leash. When you are ready unclip the leash leaving the fishing line attached. Let him think that when you unclipped his leash he was on his own. For this to work its important that you not yank on the fishing line, he must think he is free. After several moments or when he becomes engrossed in something else, call him, just once, if he responds praise him, this is what you want but if he doesn't pull him in, quickly! In this way you can remain in control and your dog can remain safe until you are certain he will come even with the most tempting distraction.