Hi, Ray.
I can certainly understand your concern in this situation. First question, has he been neutered? Has something happened in the dog park prior to his change in social behaviour i.e., was he bullied, or did another dog start a fight with him? It would be helpful if you could give us more detailed information of his behaviour and what you do, also, right up to the point where he launches into attack mode and what you do to deal with it. Start from what happens as you enter the dog park to after it's all over.
Some things to consider: Are you a calm person? Do you have him on the lead and is there tension on the lead when he is approaching other dogs. (This can make all the difference between an anxious, but non-aggressive, meeting and a full on dog fight.) Do you have a strong level of voice control over him in general? Does he see you as his leader? Being a good obedience dog doesn't necessarily mean he appreciates you as 'top dog'. Are there dogs there that trigger this behaviour in him?
Are there dogs there which he loves to be around? Does he do this as much with submissive dogs as he does with more dominant dogs? Does your dog behave like this only at the dog park or everywhere he sees another dog?
I guess, without much info to go on, if it were my dog doing this (I have a fairly strong natured Rhodesian Ridgeback bitch), I would ensure that she was iron clad on the 'leave' command, and also I would be training her strongly in the down stay. I would only approach dogs which I knew weren't going to try to dominate her, and I would put her in the down position and ask the owner to lead his dog up to her rear end (proper social greeting for dogs....not nose to nose). After introductions, I would walk the dogs calmly together for a while on lead, and then, if all the signs are good, let them play at the end of the walk (him trailing a long lead in case he needed a strong correction). When (or if) you need to correct him for his behaviour, make it a VERY strong one (preferably with a properly fitted pinch collar as they are safer on the trachea of the dog and work by 'pinching' around the circumference of the neck with even pressure). You want him to mind you no matter what. If you just constantly nag at him with a pull here and a pull there, it is more harm than good. One really fantastic correction will do more than 100 nagging ones. He needs to know this is not acceptable behaviour (oooh, I reminded myself of that British nanny lady who's on the telly when I said that...lol). He needs to know that you alone are leader and if there's any discipline to be dished out, that's your job, not his.
Don't worry, if you're consistent with gaining total control over him when he's at home and then you introduce distractions, making them bigger and bigger (and in different locations) until he is iron clad on the recall or the leave command, then you will conquer this. If your leadership is very strong, he will not feel it so necessary to step on in there...lol. But you have to be committed. All the best.
Michaela
p.s. Just be aware that if you correct him and he is standing beside a dog, it may trigger an attack. So if you do this, make it a really good correction so that he has no hope of redirecting his assault on the poor dog standing next to him.