I'm all for helping people who need it and recognizing differences in how our brains work..
But, I do struggle to see the point in a diagnosis if (if!) a person is going great. If, as you say, he is going well at school, well with friends, well at home, on track, meeting milestones, tick tick tick etc.
Then why worry? Maybe he is autistic? If so, when it manifests in a way that requires help, get help and a diagnosis. But if he is doing great, functioning perfectly and requiring no adjustments or extra support, then no need for anything else.
These days I look back ok my childhood and I remember the more "regular" children, then the ones who were slightly more different or weird in some way - including me - but could get along just fine, and then the third group that were different and couldn't get along fine, which included a few very good friends of mine.
I worry that medicalizing the second group does them no service, and actually stretches the resources that the third group so desperately needs.
All of the "weird" but perfectly functional kids - including me - grew up to be perfectly functional "weird" adults.
Maybe for your son things will change and your family will be right, and your son will begin to struggle and need support - but if so do it then, not when things are going very well for him.