Although my DD was in 4th grade when we did this, we did it and I am certain it was the right choice for her. D was in K when we moved to a school district that moved her up to 1st because her birthday fit their cut off and she was academically ready. It worked. Although there weren’t many students younger than her, I can still remember at least 3. Moved again summer after she finished 4th grade and in new school district she didn’t meet the cutoff - but was eligible to enroll in 5th because she had just completed 4th. While she was handling the work okay, she wasn’t an academic star and in new school district, she would have had kids over a year older than her due to “redshirting” being very popular here. So we homeschooled and called it grade 4b. Homeschooling worked so well we continued another 2 years and I eventually enrolled her in 7th grade. She actually plays a club sport with her classmates and would have been playing with those the grade younger based on age if we hadn’t “readjusted” her grade level.
I would totally support OP’s proposal for a “gap year” as long as she calls it something like grade 8b and allows her son to pursue what interests him academically. Yes, she will have to register him as homeschooling student, but it sounds like it might be the right thing.