I agree that you need to be around other dogs to work on the distraction. I think that taking him to the dog park on lead while the other dogs are off lead is a bad idea. Like you said you can't control the other dogs therefore your dog is at a disadvantage. Like was suggested try taking your dog on a long leash to a place where there are dogs but they are not directly in your space. Can you go to a regular park where people walk their dogs and have your dog on a long leash and practice calling him to you. That way you are getting the other dog distraction but you don't have to worry about them coming up to your dog. As he gets better with the distractions go back to the dog park with a friend and in the corner where there are not as many dogs practice calling him to you back and fourth. Start fairly close to each other and as he gets better move farther apart. The key to him coming is to be more exciting than the other dogs, you have to have better treats, love, ball whatever he likes but your has to be better than his surroundings. Good luck.