I wonder if you've given any thought to club sports (that is, those outside of 'school' and therefore organized by age/ability rather than grade placement)?
That might be a means of reassuring him that he can still have that important personal need met even if another grade skip is in his future.
My DD has 'school' friends, and she has friends via extracurriculars. Those friends range in age from 9 to 17 years old, with the schoolmates skewing older and the extracurricular context friends skewing younger.
