As a teacher I'd say that sometimes the so-called 'crappy' schools actually offer a better experience than the complacent 'good' ones in an area.
It doesn't matter that a high proportion of children don't speak English on entry to the school, what matters is whether or not that is the cae X years later and how the teachers take account of this in their planning and teaching. I've always had lots of non-English speakers join my classes (Y5/6 inner London 'crappy' school) and apart from occasional home language support offered by a teaching/billingual assistant I've not done anything very different to what I would have done were my whole class fluent in English. Children do pick up the language very quickly when they're immersed in it unless they have underlying special needs which they would also have in their home language. One year I had 7 children from non-English-speaking countries join my Y5 class. They represented a quarter of the class in terms of numbers and, one year later when they took the NC tests (aka SATs) they all gained a level 4 in reading and writing which is the national expectation for 11 year olds. As I don't speak Tamil, Twi, Yoruba, French, Polish, Latvian or anything other than English I'm not much use in other languages so I get on with what I'm there to do which is to teach a whole class, not to teach a handful of children to speak English.
If your son is going into Reception then I think it's unlikely that having lots of non-English-speaking children will be a problem. The teacher will be teaching the whole class to read and write using phonics so there will be plenty of opportunities for the other children to pick up the language without it taking attention away from your son.
I would advise against looking at KS2 results when choosing a school as they don't tell you what you need to know, by their very nature they provide a snapshot of the performance of a particular cohort on a particular day in May on a particular year. OFSTED reports aren't much use either as they are data-based and whilst schools with a 'good' intake need to do very little to impress OFSTED those schools with a more challenging intake have to jump through all sorts of hoops to ensure their achievements are given the credit they deserve e.g. if you live in an area of high mobility and pupils are not in the UK education system at the end of Y2 then even if they get a level 4 at the end of KS2 (Y6) it will not count in the value added score. Fair? No. A good way to help parents judge a school? Almost certainly not.
Go and see the 'crappy' school and ask them about the things that worry you. Meet with the head of foundation stage and ask how they ensure that all children are catered for. Ask the Head if you can see the school action plan, contact some governors, talk to some parents of current Reception children, speak to the Y6 pupils and ask what they think about their school.
Once upon time when there were no NC tests, league tables or OFSTED reports parents had to use different criteria for choosing a school... I'd go back to those methods if I were you. In fact I did just that when it came to applying for my own DD to start Reception which she has just done - she's at the local 'crappy' school and it's brilliant; she's learnt to read and write in sentences since September and because of the school's intake she gets a lot more attention that she would at the more middle class school that I now work at because at her school she stands out and is being pushed, at my school she'd just be another bright 5-year-old.