I bet your son is perfectly fine and normal.
The odd thing here, is your Ex.
Is she... mentally off or has any mental concerns?
Is your son treated well, at home, with Mom?
what is his environment like at home?
In any event, she will probably NOT like, that you are questioning her nor saying that your son is normal and not Autistic. And who knows what she will do... if she knows you are suspecting her/questioning her, about your son.
I would as the other responder said: Video him and take photos of him, doing ordinary everyday things, Like Elena B. said.
Keep it for YOUR documentation.. and also write down your observations and date it and with the time.
Take video of him playing with other kids, too.
Are you allowed to take him the the Doctor??? If so, then I would do so.
Get all the 2nd, 3rd, 4th medical opinions you can.
The fact that your Ex is going to great lengths, to prove that he is Autistic... is to me... mentally off... and even 'negligent'. Because, perhaps she is treating him as such, in order to get her son to be emotionally upset, on purpose. For whatever reason.
I would really not trust her.
She sounds to be a very... perhaps unstable Mom or personality.
My son, had a Speech delay. He is NOT Autistic at all. He was assessed per his development and speech. He was in fact, advanced in many developmental areas. He was just and only, speech delayed. And he is very bright and social and everything else, per his age.
There is a free service, which each state has... called the "Early Childhood Intervention" organizations. It is only for children up until 3 years old. But they do Overall developmental assessments on children. In my State, I did not even have to be referred by a Pediatrician. I just called them myself. That is how my son got his speech therapy.