The training should start around an environment where you have full control of the situation such as in your living room, kitchen etc. These way even the dog will be more focused to his training since also the environment around him is familiar. Once you see the dog is competent at off leash skills you can go ahead and move the training to the yard outside the house (gardened).
Use the Premack principle on him. This consists on taking a desire one has and use it as a reward to do something another wants. In this case the reward for the dog is to be off leash with all the exciting distractions such as the desired behavior, your dog coming in from time to time to check in etc.
Make them to pay attention to you. Hold your dog on the leash until they focus on you than just say ‘good’ and let him free. After doing this repeatedly the dog will learn that if he gives attention to you, they will be rewarded to go off leash.
Than to make him return to you while he wonders around the garden free and passes you by for a moment drop a treat for him and say again ‘good’ and than encourage him to go play again. Doing so he will learn that when he comes to you will be treated and the chance to go off again.
Now when he has learned how to behave around the garden, step up the training to a fenced park. Here he will be in a new environment which means that even his behavior will become unpredictable. Here is where you add new training like recall, leave it and distance downs. But being on a park the dog probably will go crazy from all the distractions around. And we do not need that so is time to use the long leash.