Every half year I get this scare. It usually happens as I read to my son in English - his and mine second language and realise what a little vocabulary he has. Today, it turned out he doesn't know the word 'ordinary'. He is nearly 7, has lived in UK since he was 3,5.
At home, we speak our mother tongue, language #1; I go a long way to help him keep it up. He goes to Saturday language school in #1; he reads, writes and speaks so that native speakers in country #1 do not recognise he doesn't live there (when we visit). Still, it is going to deteriorate, his vocabulary isn't catching up and I accept he will not be an absolutely full master of that language.
However, his English is in a far worse situation. He is quite a poor reader, his spelling is bad. I recently started also supporting his English as it was going nowhere - still very, very limited command (in my view). And he has been in this country half of his little life! Also, there is not enough time in the day to do the same amount of work in two languages - on top of his English school!
So, I am asking really for some support and any advice. Of course, I start wondering if he stands any chance passing verbal reasoning at 11+ in English, judging from where he is now... And whether his #1 is deteriorating while his #2 is not advancing... He is lost in between, lacking means to really have a full rich complex language - at least one. And rather has a little of both. Is it going to do him a favour at all?