In an interview, he talked about a trip he did in China when he was very young and he said something in French (i don't remember what). He explained it in English to the journalists and said he spoke in French with his mum. So, it means that when he was in France, even his family spoke to him in French.
When they left France, the family started to speak to him in English. That's why, sometimes, he makes mistakes. A 8 year old French boy makes a lot of mistakes because they didn't learn the rules. Don't expect him to know the rules. And we all know that now, he doesn't speak in French anymore expect if someone talk to him in that language.
I'm fed up of having the same conversation than 4 years ago. So ok, Mika doesn't know how to speak French and he should not try to sing in French because it doesn't do it well.