Hey everybody! Thanks so much for all your advice.
I have some good news to report. Today I took Henry to the park again. As we were walking along, I saw a golden retriever, off- leash of course. The dog started toward us and I did an about- face and huffing and stomping off in the opposite direction. Well, I guess the owner must have noticed my rather dramatic act and he called after me. He said, "Hey, its okay. You can walk this way, I'll put the leash on." The dog was about halfway toward us (he wasn't charging like the others have), and luckily had a great recall. The dog was so well behaved that I probably would not have minded him coming to greet Henry, it was just the principle of the thing

So the owner leashed his dog and we walked by without incident. Later on, I walked past the same pair and the dog was still leashed. I can't be sure how much the owner will stick to it, but hopefully I made him think about his manners. Either way, it was a small but important victory for me
I'm going to try to upload some photos I took of Henry at the park today too.