Finnian this happened to me when my DD moved lat year to her new school...she's 8 now and in year 4 bt then...she was just turned 7 in a high achieving year three...the new girl..and shy to boot.
Within a term, her (very young) teacher had me in there with suggestions that DD was on the spectrum,
I knew an know still, that she is not. She's bright and shy and was very new so was struggling with the social side of things...she was also two levels behind!
I refused an assessment and the HT supported me.
The teacher was still wanting it at this point.
A year later, the child who wouldn't join in, has been begging for a lead in the play and has a nice group of friends AND has move up three levels.
I had her join Brownies and made sure she had lots of playdates...I also made sure she knew that her quirkiness was a GOOD part of her.
She's like me...a bit odd in some ways but not on the spectrum. If I thought she was then I would be asking for her to be diagnosed. As it is, I have saved time and money and STRESS by refusing.
Can I suggest that in a year when your DS is more mature he will be ok. Can you observe him in the playground without him knowing? I did this so I could see what the teacher was worried about and what I saw was DD was too shy to join in.