If your son was 6 in January 2013 then he is in the right age group to start in 1st class, so he will be with kids all his own age. Even if he has done first class equivalent in England, I don't think they would move him up to 2nd class. It is much better for him to be in first class with his own age group and concentrate on the German. As a January birthday, he will be at the younger end of the year group anyway.
My daughter is in First Class right now, and she loves the Swiss school, the work is challenging enough, and they get about 10 mins of homework three times a week. The German is hard enough that I sometimes need the help of google translate! The maths so far is quite easy, but I get the impression the swiss system is going to be very good with setting a good base for learning.
We live in canton Zug, and the kids start English in the 3rd class.
My daughter can't wait for this as she is on a mission to help the Swiss kids sing Happy Birthday with the 'th' sound and not convert is to 's'! So far I have had 2 of her Swiss friends proudly show me that they can say birthday the english way!
I don't get her extra English lessons. She could read before she started Swiss school and continues to read in English, and when she is writing her spelling is not bad either.
If you do want to go the route of lessons I know there is a place in Horgen, www.fun-tastic-horgen.ch
where they run after school english classes for kids. I don't know personally what these classes are like, but my daughter attended their playgroup for a year before she was old enough to start Swiss Kindergarten and she enjoyed it there and all the staff were very nice.