Having a successful relationship with your dog is not just about training. There's a very important component that so often gets neglected, management.
What I mean by this is being aware of your dogs dislikes, limitations and triggers and avoiding as much as possible at first.
For example, does your dog act aggressively on the leash when seeing other dogs? Does he growl at new people? Does he lunge at skateboards? If so, he needs training, yes, but if he isn't in a 'training state of mind' then he's not going to be able to learn and absorb any commands and his negative  reaction time and time again just reinforces this learned behavior and 'practice makes perfect'.
Just because a nice day at the park is fun and relaxing for us, to some dogs it can be stressful and overwhelming. Thus, he may act aggressively therefore avoid parks right now.
Management is about training your dog in a non-stressful environment first. Then, over time, you begin slowly exposing him to his trigger areas (areas that cause him stress) in conjunction with the training he has learned. Therefore, with this new way of dealing with stress, he will  have a new association to that area.