It's not fair that at all, you're doing the right thing then one idiot owner ruins. That happened to us near the start of lockdown and then meant we daren't take our little boy down there anymore in case we had to have another argument with someone. It stops you being able to really relax and enjoy these places doesn't it?!That's awful! Also easy for them to say but it can affect how your son will feel about dogs in future.
I really try to point out well behaved nice dogs to my son and say "oh look isn't that a lovely dog" etc so he sees them positively and then one like that comes over (recently one was bounding around him as he was holding a stick and the owner was a bit like *shrug* 'he just wants the stick') and he will cower from the next few well behaved ones, trying not to give him the complex I had against dogs as a child(loved them when I knew them but was terrified of ones I didn't know) but it's hard!
I'm no dog expert but my job is working with dogs, I've always had dogs. The amount of bad behaviour in dogs you see and the owners literally stand there and let it happen is unbelievable. One of the dog trainers on tv made a good point, he said if your child is naughty you tell them off and prevent that behaviour, people dont do this with dogs, why? I'm baffled at what most people let their dogs get away with.