School options in London

I don't live in the UK, but my husband has been offered a great job opportunity in London. We have a 14 year old, gifted son with aspergers who is currently (after a stress break down) in a special needs class with few other students. We'd like to go to London but we're unsure of what it will mean for our son. He needs a quiet, predictable environment.

Since he started school he has missed a lot because of stress, and he didn't go to school at all from dec. 2012-dec. 2013. We're nervous that he will have to wait months again for a new school option if we move. Even though he has been away a lot, he is not behind in his curriculum. On the contrary he is ahead in some of the classes. 

I really hope that some of you can tell me about what we can expect and perhaps have some advice for us.