My hope is that when he enters middle school he will start taking some 7th and 8th grade classes while in the 6th grade. But I am still not sure if that will be the answer.
Oh boy... I could have written this myself when DD was eight...
She didn't really 'push back' on a grade skip-- in fact, she's always been more than eager to do them, but at some point, there's just no getting around the fact that the WAY that schools teach ND children is just plain not suited to kids +2SD out from the norm.
We've learned that the hard way. I wish I could tell you that a grade skip would fix it all-- but it probably will NOT, at least not in the long run. Because the underlying mismatch will always be there. There aren't enough kids like this for any 'evidence' or any standard curriculum to truly
fit them or their needs. We're all just flying by the seat of our pants here and trying to stay half a step ahead of disaster.

Your thinking about this is just so strikingly similar to what
we thought at the time that I wanted to mention that.
My advice to others that find themselves in similar situations (where NONE of the available options seems 'good') is to go with the
current BEST FIT, and then worry about things as they come up. DO NOT commit to something two or three years down the road, and do NOT agree to placement decisions that "will fit in ____ years." The truth is that you don't know it they will, because none of us knows for sure what our kids will need in two or three years-- and neither do the professionals who are trying to work with us to hammer out a plan.
Don't be fooled by thinking that middle school (or high school, or {X} grade) will be
different... it might be, but it may not be different in a way that makes it an
improvement from a HG+ standpoint. What we've learned is that sure, the output demands are significantly increased in MS and HS... and certainly the demands on executive function are greater... but if anything, that makes the fit MUCH MUCH worse. That wasn't the dimension that the challenge needed to be increased, if you see my meaning. In other words, my child's areas of strength are STILL as unchallenged/unfulfilling as they've ever been, but now she's expected to do MORE of the stuff she found pointless all along, and do it faster and with less oversight/support. This taxes her
weaknesses (related to asynchronous development) and is hard on her self-esteem in some ways-- but still doesn't allow her to stretch her wings cognitively the way we were all hoping. She's literally had
one course in her school career which actually came with some doubt about the possibility of earning an A+... and it is this year (geometry). Unfortunately, we knew we were in a race against time here, and we
lost. She feels that the fact the material isn't "easy" (ie-- I saw it once and now I have mastery, or I already knew that) means that she is "dumb" and that she is "failing." So she quit trying.
I'm not saying that I think that a grade skip is a bad idea (my DD is 11 and taking high school courses), but if that is ALL that is on the table, it may not be enough for your child. The lesson here is that unless the grade skip is going to be providing instruction that will be meaningfully different from the current ill-fitting curriculum, it may not be enough for long. At least not in and of itself, I mean. Curriculum intended for ND learners simply doesn't
fit PG students very well without a lot of modification.
Perhaps you could ask what else can be done in the way of accommodating your DS' learning differences?
My daughter's desire to remain with agemates is often based on not wanting to look/feel like a freak. Kids with really terrific social skills
may be able to 'hide' in plain sight like that. Do you think that your DS' motivations are similar? After all, he knows what a grade skip feels like. I wonder what he anticipates a second one would be like.