I don't mean to be nasty, so please don't take this the wrong way, but ever since I've been visiting mumsnet I've been aware of posts from you about your ds' education (and I don't come on that much!)
In the last 18 months, your ds has been at 3 different schools, no? For a child with possible asperger's that's an awful lot of change to deal with. He's also come across similar problems at all the schools, which would suggest to me that he's taking some of those problems with him.
Does he have a diagnosis yet? does he have a statement? are you sure the private school is really the right place for him? I seem to recall that you thought the last two places you sent him were "it".
How do you know that the prospective new school would be able to meet your ds' needs any better than the last 3 places? How does the new school view children with special needs? What sort of extra help is in place and would you have to pay extra? If your ds gets a statement, would they still accept him as a pupil? A lot of private schools wouldn't. Kids still get bullied at private schools, so paying fees does not necessarily mean your child will not suffer.
No school is going to be perfect and your son has got to learn to rub along with other people, to stick at things, to keep going when things get tough rather than run away - all things which don't come easily to a child with asperger's.
Believe me, I understand your angst - I have two children with statements and ds1 in particular can be very difficult - but in the long run constantly changing schools is not going to help your ds