Well, usually a growl like that is on account of fear. And you don't want to tell him "no" or scold him in any way if he growls or reacts because that is merely a communication that he growls. And if he becomes afraid of growling because of being supressed, his behavior will likely escalate and at some point down the road, he may explode and bite and skip the growl. It sounds like you got too close to a dog too soon. So work with him at a distance at which he is not likely to be fearful or reactive. Reinforce the way Kiko pup explains in that video.
His success at getting use to and comfortable with other dogs may be pretty slim because of being isolated from them, like I said during the first 3 or 4 months of his life. This critical socialization period is so important with ALL animals, including us that when it is missed, when the window closes, it's nearly impossible to make a lot of progress...some people get things somewhat better though.
It's a period during development whereby a young animal needs to figure out what is safe and what is dangerous. If he's gotten along all right so far without something, then he pushes that aside as nothing he needs to survive. For instance, if the puppy never sees another dog, he has no use for them later in life because he's survived this long without. Who needs 'em? Another dog is not part of his repertoire of safe things. Dangerous things go in the "box" of things to stay away from and that's what happens to things, dogs, people that the puppy has had no exposure to. Safe things are those things which he uses in day to day life and has had plenty of exposure to. He has those needed things in his life and therefore, they're saved in the "box" of safe things. For instance, in your dog's case, you. Or the lawn or the other dog in your household or the TV blaring and whatever else he's experienced and been exposed to happily since day one. He feels safe with all the things.
At 14-16 weeks of age, this "window" (birth to about 3-4 months) of figuring out safe and dangerous
closes so a baby animal can get on with the business of survival and living into adulthood that can reproduce and pass on his survival genes. (instinct) If that weren't the case, you'd have adult deer walking right up to you in the deep forest. So, that's a simplified, nutshell version of the early critical socialization period or sometimes called the sensitive period.
Here is an informative article about socialization from a wonderful behaviorist, Sophia Yin (RIP) who tragically took her own life. She was always my hero...so smart and expert in her field.
https://drsophiayin.com/blog/entry/puppy-socialization-stop-fear-before-it-starts/
Of course, your dog is an adult so this is especially good to learn about if you ever get another puppy.
But since your dog is an adult, you can try counter conditioning in a positive reinforcement way like described. But if your dog doesn't make much headway and he's uneasy, frightened, defensive (might bite out of fear) you'd be best to leave him in his comfort zone at home and forget taking him to ball games. Those can be pretty noisy and stressful even for relatively well socialized dogs. At his age, I'd give it a try if he were my dog as far as helping him get more comfortable. I'd play it by ear. And if it was too difficult, I'd figure it isn't worth it and let him live out his life in his comfort zone that he's known all this time.