Is this normal behavior, is there anything i can do to prevent this? He is literally the most loving dog and all the kids cuddle with him and pet him. He is good with all dogs. But like i said with those 3 dogs as soon as he smells them he changes instantly.
Now would be a good time to start correcting your dog. Look for the signs that he's challenged by the other dog. As soon as you see that sign, correct and make him sit or down or both.
And if the other dude is fine with his dog getting beat up, then that's on him. Not your problem anymore.
I hate it when owners say "boys will be boys"!! In our early days, I took my dog to a dog park and another dog kept trying to hump him. My dog turned around and growled at him several times. I took his leash out of my pocket and got ready to leave, but the other owner said, "Boys will be boys. Let them work out who's boss." There was no way I was going to risk a dogfight or vet bills! I grabbed my dog and left, feeling pretty annoyed that the other owner didn't train his dog to have some manners.
Jack also growls at other males that challenge him, and if left to his own devices, would prepare for fisticuffs if the other dog made good on that challenge (actually started something). So, on the leash or off the leash, we avoid other males that challenge. Not worth fisticuffs, especially on a leash. He's normally glad to take the high road anyway.
Regarding the dog park thing, the humping is incredibly annoying and hard for an owner to stop. It's like once they start, they're set on continuing to hump. Only thing you can do is remove them or move them further away.
But I have seen it where two dogs (usually males) will growl and scuffle for a few seconds, then the matter is settled. This happened to Jack. A smaller dog at the dog park kept nipping at him and pestering him. Jack decided he'd had enough, growled at the other dog and when it didn't comply, he pinned it to the ground. From that moment on for the rest of the visit, the smaller dog left Jack mostly alone and when they did interact, it offered appeasement behaviors instead of rudeness. Jack, in return, completely ignored the bully dog. All was well.
Could it have gone sideways? Yes. Like most social animals, dogs that have developed a good "vocabulary" rarely actually fight. It's the ones that never learned how to speak dog or are mentally unstable that cause severe problems.