You can always use a long leash up to 50 feet to give the dog the feeling that he is off leash while playing at the park and giving you some control on his movements at the same time. You just need to work on leash management skills and always paying attention to him.
Now before letting your dog off leash entirely, reinforce the training you gave to him in your garden and remember the most important skill is the recall when it comes to reliability. Continue to reward the dog anytime he returns when even when you haven’t recalled him.
