Ok, - being kept in for lunchtimes for a week because of misbehaviour (one incident) is normal. (although I always feel it is a bit counter productive, keep an active child in all day and by the end of the day they will be misbehaving in the classroom)
Being kept in every lunchtime (only allowed out for 10 minutes) because school cannot manage his playground behaviour - this is not normal.
His well being must be part of the discussion. In this case he needs exercise, and playing is good for physical development and co-ordination.
He needs to learn to socialise, and play appropriately with other children.
I suggest you email school/teacher (start with teacher) and put those 2 points. You understand he is struggling with playground behaviour, so what plan is the school coming up with to manage it, teach him, and still facilitate his need for fresh air, exercise and social interaction.
If this is an on-going issues, then keep a paper trail of all meetings/discussions with the school, literally in your diary write - spoke to the teacher about x, reply was y.
if teacher is no help, ask to see SENCO or Head, and talk to them.
Be positive and look for a solution - what can we put in place to get him outdoors and safe and socializing?