A big fat NO WAY on the dog parks. Leerburg | Dog Parks: Why They Are A Bad Idea
That is a decent/good article, but it must be added that GSDs also tend to play very rough...other dog owners can be quite put off by it. Also, ONE bad experience could very well give him lifetime issues.
Your little guy needs to really get out n' about! His reaction is fear based due to the submissive peeing. At least that what it seems like. If you can (I know times are tight) try getting a GSD experienced trainer to guide you and your pup.
If a trainer isn't in your financial or time budget...
Introduce him to a very playful and gentle dog. Then as many more as you can. This worked magic for me, as I could not afford puppy class. If you can afford puppy class, by all means take it!
His behavior can become a problem...there are a few things I would recommend.
1) Introduce him to as many people and dogs, in as many places, as you can. Always make it positive, and back off if he is overstimulated. Find a reward that reeeaaaally floats his boat. Use it LOTS.
2) Find that golden reward of his (pieces of steak or cheese, a certain toy, playing tug w/you, etc.) and let him have that whenever he looks at you. Build this up nice and solid, gradually increasing the time he looks at you before getting rewarded.
When you feel more comfortable, do this in more places of your house. Then the backyard. Then the front yard. Then the front yard and out on walks with distractions far away. Then keep building until he stares at you when something weird happens.
Easier said than done, I know :P
Things that helped me n' my pup: Introduce to other playful dogs, playing tug and fetch on a long line in my front yard while the world whizzed by, carrying food with me and counter conditioning, and time.
A lot of this could very well just be a fear period.