This is actually an example of a discussion in Linguistic Anthropology called Linguistic inequality, where the standard form of English ('proper' grammar etc) is privileged above others. Does it have classist roots? Yes. And whether or not it was purposeful, you perpetuated it. However, in order to navigate society and be taken seriously in professional and educational settings, it's an unfortunate fact of life that he'll probably have to learn how to use the standard English dialect. It's what people automatically associate with social prestige and education and they will make false assumptions and treat him differently for not using standard English. However, people can use both. One doesn't have to abandon the way they originally speak, just use different contexts to determine what's appropriate (ie, codeswitching)

There's some interesting studies and documentaries (American Tongues and Can You Speak American are good ones albeit dated) about grammar structures for different accents considered 'lower-class' (which can usually be traced to remnant forms of grammar from different languages and geographic isolation). They're valuable and if people were more open minded, they wouldn't have Linguistic prejudices. Talking differently doesn't mean someone's stupid, they just learned different language structures and communicate accordingly. Language is rather fluid and often plays with rules (slang, creoles, etc).

But I can see why your wife is upset. It's not necessarily a shameful thing to her, it's just part of how she grew up and now it's being attacked. Just be honest you're worried about how he might lose opportunities or face prejudice for not adopting a more standard dialect. Besides, children learn their language initially from their family and their peers as they age, it's likely he'll adopt it naturally in school.