How well do you know the parents? For example, what are their fields of work? You say they have dismissed concerns in the past - did they say why they felt that your observations did not match their own?
I ask this because I was a caregiver for some time with a boy with classic autism. Mum was a professor at the university, dad an elementary teacher. The only language he had was entirely echolalia, not scripted, but immediate and repetitive direct echoing. He was not toilet trained (or even close) by the time he was 5, had very restrictive interests, multiple sensory behaviours and encompassing obsessions. His parents could not see (and were not able to see) any of it.
I spoon fed it to them - finally, after over a year of discussion, they had him assessed - and were surprised to hear he had autism. Sometimes, the "it can't happen to us" is very pervasive. Sadly, because of the delay in accessing diagnosis and treatment, he has really struggled over the years.
With that said, perhaps if you don't have concrete examples, it might be advantageous to wait, and document your concerns in a more quantifiable way. With that information, the parents might be more receptive.
On the other hand, perhaps the parents are dismissive because they are legitimately not seeing what you are. For example, my DS was selectively mute at school for years, but a chatterbox at home. If someone told me he never spoke, I would have brushed them off, because to me, it would have seemed bollocks. As a teen, I never spoke at home, but with the right group of friends I was an eloquent speaker.
If this child is an anxious child, perhaps the things you are seeing are school anxiety, and not developmental in nature at all...
Just my rambling contribution...