
I actually like mixed schools :) Both my son and daughter went to a mixed grammar school in preference to single sex schools and think it has helped them to get on with the opposite sex without finding interraction with them a big deal. My son has lots of friends who are girls - still keeps in contact with them even now, and my daughter also has a lot of friends who are boys :)
Interestingly enough, when my daughter was still at school, the school which had the most teenage pregnancies amongst it's pupils was a girls grammar school. Teenage years are difficult and I think, for our family, that attending a mixed school made the opposite sex no big deal for both my children. My son, who was prone to only doing the bare essentials for his academic progress, actually worked harder at his mixed school because he didn't want to be beaten by the girls.
Hopefully your son will settle in quickly at the school he has been given :)