Looking for a tutor for 9 year old child with Aspergers

Hi there

I have a 9 year old son that was diagnosed with Aspergers 3 years ago. He is in mainstream schooling and doing reasonably well but he very much struggles with Maths even though it is clear he is an incredibly intelligent child. I think he could do with some extra support and would like to find him a tutor that specialises with children on the spectrum, to come one evening a week to us at home. I just don't know where to start looking for someone that works with children with Aspergers. I live in NW London. Does anyone have any advice?

Thanks!