Even after the grade skip, my son continues to do some activities with age mates (sports are grouped by age and not grade in school in the community and scouts we kept him in his scout group by age even though you can go by age or by grade according to scouts...we let him choose and he wanted to be with his friend)...but has also done one activity with high school kids and kept right up/fits in.

I guess I am just not as worried about the social as much as some people might be because he is pretty mature and socially advanced with just a bit of whiney little boy reserved just for me when he is tired or hungry. Also he has a December birthday so he isn't too much younger than some of his classmates.