My puppy is a bully and gets bullied

My almost 8 month old male GSD/Malinois is pretty fearful of dogs and people and I think he reacts by trying to seem tough when he sees an opportunity.

I've had him since he was 10 weeks old and immediately began taking him to my friend's house for doggy play dates, I also have an older dog and he's gotten along with her great. We're also sitting our friend's border terrier (obviously much smaller) and they've been playing great. But at the dog park, he's kinda a jerk. This isn't your traditional dog park, it's over 100 acres and I go between 5-7am when there's less than 20 ppl. But my dog usually avoids other dogs that come say hi to him or if they run at all, he will chase them growling and barking. It looks and sounds awful but he has zero follow through, he doesn’t continue to chase when they stop etc but it alarms the pet parents (understandably) and an older man tried to hit my dog with a stick this morning so I really want to nip this in the bud. On the opposite end, if a dog tries to chase him he'll scream like he's being stabbed and even sometimes pee.

I feel like all of this is him being scared and not confident. What can I do to boost it? Is he actually aggressive to the point I should avoid the dog park? Should I keep taking him as a form of exposure therapy? Is it just his weird teenage stage and he'll grow out of it? He's my first mal but my 11th GSD which is why I felt confident in getting him but I've never had a puppy that didn't want to play with ppl or dogs. And I don't care if he doesn't want to play but he does need to act right. I'd like him to just react calmly when greeted and confident when confronted. I said when I got him that I'd get a trainer if need be but this behavior has come at the worst time. My other dog just had an expensive spleenectomy less than a month ago and I'm also having surgery in a couple of days so money is really tight right now. Any advice is greatly appreciated.