OK. You have a dog who is not an instant 'loves every dog' dog. It happens. I have one of those, too.
PLEASE don't force your dog to go to dog parks and then jerk on his lead if he growls. PLEASE.
You are teaching your dog that you will not protect him. You are teaching him that he can't trust you to look after his interests. You are teaching him that you will punish him if he lets another dog know that he's not happy or comfortable.
This is really dangerous. It can end up with your dog biting another dog unexpectedly because you have taken away all his alternatives and left him feeling stressed and unsafe with no other way to make the scary go away. Or he might bite you, because his world is no longer a safe place.
Socialisation is important, but socialisation doesn't have to mean going to a dog park where dogs can approach your dog closely offlead. It can mean walking past other dogs at a distance on lead, it can mean standing 300 yards away from a dog show where your dog does not feel he will be rushed, it can mean training at the other end of a field while someone else works with their dog within eyesight.
Some dogs are the equivalent of human introverts. They find the company of strange dogs stressful and wearing, and they can only take so much of it in a day.
Never reprimand your dog for growling. If your dog growls, he's telling you he's uncomfortable. Accept what he's telling you and adjust things so he feels happier. Often this will mean walking away. That's fine.
My credentials for this advice: I have been involved in dog rescue since 2005, I have fostered many rescue dogs, one of my own current dogs is a nervous dog who behaves just like this, I have consulted a qualified behaviourist for help with this and with previous foster dogs, with good results.
PLEASE, when it comes to taking dog advice, be cautious of asking questions on the internet that may be answered by people who are taking tips from watching shows about dogs on TV featuring people without behavioural qualifications.
Here is an article from a very well known behaviour specialist that may help explain how your dog feels :
Here's an article explaining 'look at that' training, which may help you :
Here's the official home of the Behavioural Adjustment Therapy system for dealing with fear and aggression in animals, from Grisha Stewart : http://empoweredanimals.com/
Here is an article about dog park etiquette from an industry-recognised expert :