I think that the key is your particular child is very stressed by having to choose a seat, whether that is "reasonable" or not. If you don't think that you can counsel your DS to help him become comfortable with this process, then your only option is to ask the teacher to assign him a seat. I would not criticize her practice though since other kids don't have issues with it. You need to focus on how upsetting this process is for your DS and perhaps enlist the school counselor's help.