With all that said and I completely understand where you are coming from and why, do you feel this way about intact male dobes or all dobes at parks. We will be getting him neutered, most likely around 1.5 years old (wanted to wait at least 1 year) before doing so.
Same sex aggression is very common in male Dobes--common enough that with many years and many male Dobes I always assume that beyond about 6 months my dog, no matter how well behaved he may be is unfortunately likely to be involved in a fight (more often than not, not of his own making) and no matter who started it he's the dog who will be blamed.
About the dog that your dog reacted to--were you watching that dog the entire time he was approaching your dog? It's just as possible that the other dog offered some behavior that caused your dog to react the way he did. Many males (neutered or not) are masters of the "stink eye" or vibrating tail--both aggressive behaviors--and can get a big reaction out of another dog and often none of the owners saw what started it.
While a neutered dog may start a fight with a male "just because" it's equally common that intact males don't like neutered males and they can do the same thing.
And even in small parks with low volume of dogs all it takes is one whose owner isn't paying attention or whose dog is giving off signals, to start a fight--on leash I don't worry about this sort of thing. Go to any dog show and look at male Dobes waiting at the ring entrance--the owners, handlers and the dogs are pretty much always aware of their dog and what it is doing and you rarely see any sort of aggression much less out and out fights even between breeds more aggressive than Dobes (take a look around the terrier rings sometime). But all of my dogs learn early that aggressive behavior, on leash, just isn't going to be allowed.
And I play the odds and simply don't take any of my Dobes to off leash dog parks. I too had one, and only one male Dobe who could be put into a yard with any number of dogs (Dobes mostly) of any age and sex and get along with all of them--it gave me heart attacks just thinking about it and I wasn't the one that was doing it--it impressed the people who saw it but it just wasn't safe. I stopped it as soon as I found out it was going on.
Good luck--just keep training him to be polite and disinterred in anything except you while on leash.