I am a parent of a daughter in Year 1 at Belleville, and my son is about to start nursery there in September. I agree with the previous poster that Belleville is a great school, and definitely has more clubs / activities / academic rigour (in English) than Wix at the moment.
However, if your objective is for your son to learn French, then I doubt that Belleville will help him much. I understand from other French parents at Belleville that French between the commons is suitable for kids who already speak French, not for those who don't. So, if you choose Belleville over Wix, you will most likely have to give up on your son becoming fluent in French, but you will gain him going to an excellent school instead.
What are your long-term plans - do you plan to continue living in the UK (or any other English-speaking country) and pursuing his secondary and tertiary education in English? If yes, then Belleville might be a better option than a French-only stream at Wix, but at the expense of his French.
I do know other family whose kids go to Wix and they are very happy there (the family are French speaking), and they take the kids to extra curricular activities elsewhere (e.g. at Belleville), but you have to do the pickup and dropoff (not a big deal in the grand scheme of things, of course!).
Also just wondering - does Wix provide any help for kids to pursue the French stream, who don't speak French? Just wondering if you are planning any catch-up / camps for the summer so your son isn't completely lost on Day 1.
Best of luck - whatever decision you make, I am sure that your son will get an excellent education.