I started a new school away from primary school friends when I was around your son's age and was extremely miserable for the first 6 months. This included not wanting to get up for school, feeling ill, begging my parents to let me change to the same school as my old friends etc..
My parents gently but firmly refused, telling me that there was no guarantee I would like the other school either and that I could still see my old friends without being at the same school.
It was hard and I was very unhappy. However, in retrospect, I think they made the right decision.
The school I went to was a better school and gave me more opportunities than I would have had at the other school. I know now from my old friends that bullying and drug-taking were common at that school whereas they were rare at my school. I was also forced to make an effort to maintain contact with my old friends through letters and telephone in a way which served me well for professional networking in later life (I was quite introverted and this didn't come naturally). I'm still in touch with my closest primary school friends today.
Most importantly, I learned that I was largely responsible for my own happiness and it didn't depend on being in a certain place with certain people.
So, my experience was that, as long as there's no direct cause (bullying, victimisation by a teacher etc..) eventually the unhappiness goes away.