Are you interested in private English/bilingual kindergarten/pre-school or just outside of school activities?
For the former there is
Kidscamp and the
Early Learning Association. Also the international schools: ISB and SIS as well as a few others.
Kidscamp also run school holiday clubs which are great - so much fun - in sports, arts and crafts and music. My son has been on lots of these and always enjoyed them.
Ahead With English mentioned before are great - both my kids are now going there and love it. Great for learning/improving reading and writing skills (and also just interacting with other English speaking kids).
If you're interested in sports there is
Intersoccer - which runs English speaking football clubs - after school, weekends and during school holidays. We've not used them but have heard good things.
If he's going to Swiss primary school this year then there is also the HSK who offer culture/language courses for non German speakers - I think this is only available for mother-tongue languages though so if English is a second language for him then it might not be applicable.