Excellent point for the kids that repeat 8th grade to look better for high school. Burn that year and there goes a year of juniors which could make the difference between D-3 and D-1
You burn the year when you think he needs it. Athletically, academically, emotionally, etc. In the end, the kid will develop and land where he belongs. There will be times when he seems well ahead of his peers and times not so much. Later on he will be playing with kids two years younger, two years older.