Unsupportive Grandparents
Sooo, yeah, my parents were not super supportive. They blamed me for my daughter's autism and claimed it was a certain drug for migraines I shouldn't have taken while pregnant. They watch my daughter daily while I'm working (she's 2 years old), but won't allow any of her therapists to come there to work with her, which would have helped a lot. They also regularly make comments about making sure I don't take any medications while pregnant (I'm pregnant with our second). And you get the picture...
In most parts of the world, slapping a kid until he or she shut up was not considered abuse during that time.
What finally ended the years of my MIL telling me how to raise my son was to drag her to a medical conference for our rare liver disorder (his autism is a co-morbity) and have her sit thru a few family teaching sessions. Then I introduced her to my son's specialist who heard my concerns about MIL's opinons. He then greeted MIL with, "Oh. So you're the ONE who will not support your grandchild's health protocol?"
She looked like the ground had just swallowed her whole.
After that, we buried the hatchet. And she was the biggest supporter for my son with diet and regimen. She passed away a few years ago and we miss her dearly. I do not miss the old POV she used to have, tho!
Here is how I handle unsolicited/unwanted parenting advice: "Thanks for the feedback. I appreciate your concern. I will take it into consideration." Then I promptly consider and discard it. It doesn't matter if it comes from friends, family, or strangers.
I find that trying to make people come around tends to make them dig their heels in. I think that it is even more true for someone with some neuro-based rigidity. My dad is either on the spectrum or such a big shadow that he might as well be, and while he is generally supportive (in that he keeps his mouth shut), he has sometimes said things that annoyed me. I chose to let them go. Over the years, I think he has come to understand that my kids are "different," and although I still don't handle things the way he would, for the most part we have an implicit agreement not to discuss any of it. My mom knows a lot more, because she is more receptive and I just feel more supported by her. Plus, I do think that part of the issue is that my dad knows I think he is on the spectrum and although logically he probably can't argue because his "quirks" all line up with a fairly typical Aspie presentation, I don't know how comfortable he is with the idea as a whole. Because if he admits that he has issues that he can't help, then that means that my brother and I probably also couldn't help the issues we had. And then that leads to the realization that he did some damaging things when we were kids.
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Mom to 2 exceptional atypical kids
Long BAP lineage
I wouldn't discuss it at all with your dad- maybe just your mom if she's more reasonable. All the stuff they did to you is absolutely horrible parenting. Seriously wtf they are giving you terrible advice, regardless of your child's issues. I don't think there's anything you can do to change their minds, especially since it would mean admitting they were substandard parents at best. No one likes being in the wrong.
I would shut the conversation down any time they brought it up. Make it an off limits topic. Tell them flat out you don't want any advice from them on parenting and if they ever try to "discipline" your child they won't be allowed time around him.
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AQ: 35
Your neurodiverse score: 119 of 200
Your neurotypical score: 88 of 200
You seem to have both neurodiverse and neurotypical traits.
Parent of an autistic son.
If you and your child both had similar problems, and you are pursuing a very different style of parenting, then that is inevitably a judgement on your father and his behaviour. I am sure you don't mean it to be so, but it is, and he may well see it that way. That feeling of being judged and found wanting may cloud his ability to be reasonable. I don't think you should change your parenting, you have obviously carefully considered your child's needs and behaviour, but maybe it is too much to expect that your father can be totally happy about it? Essentially, you are telling him by your actions that he got his parenting badly wrong.
good luck to you and yours!