[NikkitaZ]

Answer this for me: why would you take a known dog aggressive dog to a DOGPARK?!
that is just being awfully irresponsible, and asking for more Breed Specific Legislation (BSL) and even more apbt's and mixes killed.

NEVER TAKE AN APBT OR BULLY MIX TO THE DOGPARK!
in fact, never, EVER take any of your dogs, regardless of breed to a dogpark. it's NOT a good socialization opportunity, it's only good for your dog to catch some nasty diseases from other unknown dogs and get him into fights with other unknown dogs.

Now, Dog Aggression (DA) and Human Aggression (HA) have NOTHING to do with each other. They are worlds apart from one another.
One doesn't lead to the other, and especially NOT because your dog got into a fight with another dog.

there is something wrong with your dog, wether he is really HA, Fear Aggressive (FA) or very territorial,...
I'd suggest you to find a certified behaviorist and work from there.

HA is not tolerated in this breed (however, I would keep a HA dog if he didn't show any signs of aggression to the people that live with him), and if you still want to keep him, no more walks outside of the house, only playing & exercising in the yard, put him away EVERY time you have visit, putting up a WAY bigger fence, chainspot (because he could still easely jump a 8ft fence if he really wanted to etc etc
at least until you get a certified behaviorist that will tell you what exactly is wrong with your dog, and if it's workable or not, and then it will depend if you want to keep a dog that could cause much harm to your commutiny, and if you CAN own such a dog responsibly, without putting your community at risk of serious injury.

you won't fix his DA, if you don't want to invest time and manage it instead of waisting your time trying to "fix him", this is not the right breed for you.
he is not a bad dog because he is DA. he acts upon his natural breed instincts, just like a JRT isn't a bad dog because he'll kill all small game he can get to.

I'd also suggest you invest in a Breaking stick and learning how to use it, you now know how hard it is for them to let go of another dog.
a break stick will help you pry his jaws open to release the other dog once he got a grip and won't let go.

sent a PM to Colbydogs or to Zoe, they sell them (if I remember correctly )