I have a 1 year old heeler pointer mix, Bosco. He is toy agressive with other dogs and only with a tennis ball. HE can play with other dogs with a tug of war rope, any toy but a tennis ball and only with other dogs not with people. He obsessive with the ball. He has attacked other dogs at the dog park and even my other dogs because they had a tennis ball. I have since stopped bringing him to the dog park but he still agressive. I do not know what to do. he was socialized as a puppy and he lives with other dogs. he used to be fine and he played well with all types of toys. Any tips on how to get him from doing this agressive and dangerous behaviour? Any tips would be much appreciated. Thank you.

Did you ever consider to stop using tennis balls to play with the dog(s)?
See what happens then......

The problem is not the tennis balls, its the fact that he attacks other dogs when he is playing with the balls. Fetch is his favorite game and my other dogs enjoy it as well. I want to be able to play with him and with my other dogs without it escalating into a fight. I am afraid that if this continues, it will get worse.

If he is really only aggressive with tennis balls and NO OTHER TOYS, then only play with the tennis ball when you are alone with him. Put your other dogs inside and play alone with Bosco. This would be an easier solution than to get a behaviorist to figure out what is going on.

First of all, don't put him in a position where he can fight with other dogs over it. Next, go to a quiet place and give him the tennis ball. Then take it away and give him a treat. Then give it back. Once you do that he'll start associating the ball being taken away as getting a good treat. Some dogs are just going to be possessive over their toys. One of my dogs will let any dog go near her when she's got a toy, my other dog won't stand for it.

Thank you. I will definitely try that. He gives me the ball just fine. He wants me to constantly throw the ball and he will keep going for hours! He just won't let other dogs take the ball. I just want him to be able to play with all other dogs and not fight. Like I have said, he has attacked other dogs because of this; he was not playing. It was an all out, I'm mad attack. When this happened I didn’t even know he was having issues with the ball or other dogs. I tried to play ball with him and my other dogs but he attacked them also. I have since stopped going to the park and he is not around any dogs except my other ones. And before when I would take him to the park, he was fine with other dogs. He would share all the toys with them. I do not know what happened to make him act the way that he does. This is why I find it so puzzling that he does this. I am afraid that it is going to get so bad that he will be possessive over all toys. I know that I said that he only does it with the ball but that is right now. I am afraid that if I just stop playing with the tennis ball altogether then he will just find another toy to be obsessive about. Thank you again. I apreciate the help.

I am not sure this helps but.....
When I had 3 dogs at one time, I made sure I would train each individually and spend equal amounts of time playing with each. I would let the others watch. I either had the other dogs tied or crated but they had full view of what I was doing with the other dog. BTW they each had their own toys. I would make sure to play with them with their toys.
I also had the dogs do things together. Walks and car rides and sleep together. Tug of war with each I did allow. I was careful about the wrestling.
Some things that you mention about not going to the park seems to be about social skills. I think what is needed is a different approach to being out in public.
First make sure your dog is good at heel, sit and down commands. If this basic obedience is good. Then make sure the dog is good at recall.
If all those basic skills are good, try to bring your dog to a public area that is not too busy but does have some activity. Do your basic obedience skills training in that area. Make sure the dog is safe in this public area. What you are doing is getting the dog ready to go to the park where it should obey you no matter what else is going on.
I do not think it is written anywhere that a dog has to share their toys. After all they are not children or adults. They are dogs. Their instincts form their behavior.
If your dog becomes possesive of all toys then you have a dominance issue. This is more prevalent in male dogs.

Thank you very much! Bosco is good at all of his commands: sit, stay, here, down, heel. Taking him to public places will be very helpful. Thank you again.