We have exactly the same situation with ds1 and his best friend.
His friend is nt, very popular and really looks after ds, making sure he's included by the other children in the playground, explaining bits of the lesson he's missed etc. He does see ds1 as his best friend, but not in the same way as ds1 does. Ds is totally and utterly smitten/obsessed (not sure of the right word really) with this friend and we have weekly problems when the friend has extra curricular lessons at lunchtime twice a week and ds can't integrate with his peers without his friend facilitating it for him and similarly if the friend is off sick.
According to his mum, the friend worries himself silly about how ds1 will cope if he has to be off school for any reason and insists on going in when he's ill as a result. He also frets about what's wrong if ds is off.
We have been trying to address it, but its really hard to know how. His class and inclusion teachers have tried to widen his social circle, working with him to identify other children he likes and shares similar interests with and encouraging them to work together in class. He did start to build somewhat of a friendship with another boy, but then we found out his family is moving to the other end of this country this spring.
His teachers also put a ban on him working with his best friend in class for half a term and encouraged working with other children in his class, but it had little effect.
The EP wants the school to set up a lunch time club, mainly to work on his social skills in general, but also to try and build bonds with a small group of 'vetted' children. She actually wants this put on his statement, but as the inclusion team have talked endlessly about 'circle of friends stuff' that's never happened, I'm not holding my breath.
We are now worried about what's going to happen come secondary school, as we have to name a school in the summer term and won't know whether or not his best friend is going to grammar school or the local secondary. I can't see ds wanting to go anywhere without his best friend, but we might have no option. 