I work with an autistic child and two children with suspected ASD, and I would say if your child has any kind of ASD or any other "condition", diagnosis can only help. Yes, people are obviously wary of "labelling" a child, but unfortunately in this country, you will not stand a cats chance of getting him any help he may come to need now or in the future with out that label - wrong, I know, but that's how it is.
To rockolo, please remember that teachers and support staff are not necessarily being "critical" - they want the best for each child in their care. I know of none who will highlight a child unnecessarily - it is not an easy thing to tell a parent, just as it is not an easy thing for a parent to hear, so believe me, no one does it if is not needed.
aims, please try not to worry, talk to as many people as you can and work with the school, don't pull against them, then whatever your child becomes, he will have the support he needs. But don't forget also, that this could simply be behavioural, a period of adjustment, and again with the right support, he can adjust to his new setting perfectly well. Just keep an open mind (which you sound like you have already). Good luck