I wonder if this will work for him at the dog park. He hates it and drools excessively. I know dog parks get a bad rep, and I don't want to use the shirt just to I can take him to the dog park - but just so he's not stressed in new environments like that. He also drools at the vet's office.
No reason it shouldn't. At my office, everyone with a dog ended up getting a thundershirt for a variety of reasons within about a month of each other. And everyone gave it rave reviews. It's expensive, but they have a money-back guarantee.
I'm a big fan of the dog park, but we have a small dog park where I'm at. My guy loves it. However. it took a lot of socialization to get him there and confident. We initially started with puppy play sessions that a local trainer monitored. As a puppy, he always hid behind me at these events, so the trainer put him in a expen for the first couple of weeks to watch the other pups having fun. She also didn't let me comfort him (reinforcing his scared mood). After about 3 sessions, he seemed interested in the other dogs and she let him out at the end - when everyone wasn't quite so rambunctious. By the 5th session, he was running around, wrestling having a blast.
That said - he is still one freaked-out little guy at the big dog park. The first time we went, he was confident and tried to play. He engaged one dog, but the next thing we knew, there were about 5 big dogs playing. He accidently got stepped on; and that was that. The big dogs are much more rambunctious, and the owners seem in less control (ie not quick to leave with a dog that starts a fight). Other people have horror stories of a big dog seeing their little guy as prey and just going for 'em. So if you have a mixed sized dog park, I'd be weary of forcing the issue with your guy. And lots of socialization plus treats/rewards in any new environment.