OP, I think it really depends on the ethnicities and the school you are talking about. People on here are generalising, talking about various different ethnic groups and different situations that can arise, but they not your situation, which is what matters here.
In my area, all the schools are predominantly one ethnicity. If I could find a secondary that was as mixed as 80%/20% I would be delighted. They are mostly made up of in the mid-90% one ethnic group. My daughters will be going to these schools....the only options are unpleasant church schools that seem really rough, not very churchy, and populated with white racists who are there for the sole reason of avoiding the Asian population.
I am getting dd1 involved in out of school clubs so that, in the event that if she only makes school friends with people who aren't allowed to socialise with her, then she will hopefully have people that she can go to the shops/cinema/have sleepovers with, if she wants to.
I visited schools last year, and when able to I asked outright about how many white children there were in the school and how they fitted in and were there any problems etc. Maybe that is something you could do - try to get hold of a student to get their point of view? Visit on different occasions to ask different people?
Academics aren't everything, your son will get the results that he deserves from the work he puts in. What are the results in the goodish comp? What else do the schools offer in terms of music/facilities/sports? Take all that into consideration too. How bad is goodish exactly?
Also, how far are we talking to the goodish comp? I used to travel 45 minutes door to door, and I took that totally in my stride. It was a fun part of my day because friends did it too, just more time for chatting. Something to consider also...if he goes to the further away school, will his friends be from further away so he will still be in the situation of not having local friends to hang out with?
Lots to think about...oh for a crystal ball.