In our public schools, many of the boys are held back. So my late May birthday son is one of the youngest in his grade. There are many boys a year older or even more in his classes. He was among the last of his friends to drive and he is the “correct” age for his grade. My son has always been very happy socially so I don’t regret not holding him back. But there were times in grade school when the age difference was difficult for him because of classroom expectations.
My point is that if your son repeats a grade in his new school, he still might be among the younger kids in his grade, I’d ask the school if boys are routinely held back. If so, then having your son repeat might be best.