In my mind, here's the thing: this is preschool. If his social development has not caught up with his intellectual level, is preschool causing him more frustration than it is worth? Maybe preschool isn't necessary.
When I was studying special education back in the 80's, two professors at my college were in the midst of a study on summer children, and the statistics pointed to something like 70% of the boys being held back by grade 2. Girls were not nearly as affected.
But gifted kids have the added frustration of being intellectually ahead of most of their peers and even those older than them.
We enrolled my older son in Kindergarten when he was barely 5 - made th e cutoff by two days - and while there were a few challenge (much physically smaller than other boys in high school, some immaturity issues in mid-school), I'm not sorry we enrolled him early. It was a "grade skip" without the battle, and he needed it academically.
But we also kept our kids home until Kindergarten. My son had 5 full years to be "normal" at home before having to contend with his differences in the classroom setting. I think it was a big boost to allow him to develop at his own pace at home, and I think the lack of stressors of not fitting in in preschool helped him actually be more prepared for Kindergarten.
Just my thoughts based on my experiences, for whatever that is worth. (By the way, his toughest year by far was as a freshman in college. As a sophomore, he's finally gotten his groove and is loving the challenging engineering courses.)
