Congrats on your son's placement, and heartfelt congratulations on having a "twice gifted" child.
My recommendation would be to have him in the G&T program but also have him in some kind of social skills group -- some program for autistic kids, so he doesn't feel like he's the only one.
I personally am from the school of thought that holding a child back a year is not a panacea. If you put him with a younger group of kids, their social maturation will still be different, and in a sense more rapid, than his. He'll just stick out more b/c he'll be taller. And the academic work will be less challenging. For a kid who loves to be challenged, that doesn't sound like a fit.
What I would really recommend is sitting down with the teacher, possibly the principal, and any specialists the school has -- and coming up with a proactive plan to help the kids understand that "Johnny" speaks differently than they do, but he's really smart and really good at X, so it's not okay to tease about speaking, ever. Don't assume the teacher and the kids and the school will figure this out on their own -- but you might be surprised about how responsive people can be if you take the initiative.
And, finally, a lot of highly gifted kids are quirky -- maybe not enough for an ASD diagnosis, but enough that your son may feel more at home in a G&T group than you'd expect. That's my son -- years advanced academically, several years behind, in a way, socially. We make it work ;)