I'm very surprised that someone suspects autism at 18 months. That is too young to assume a child is autistic. He may be too smart and independent and choose what he wants to do. My little one didn't say anything other than dadaaa at 18 months. 2 years, and every day, she says more and more words.
It is early to worry. That said my step daughter is autistic and her first words were Thank You (which she would say when she took sonething from you) starting around 18 months
I'm surprised your 18 month old has been flagged as having a speech delay. Is that by a professional or is that your assessment? My eldest didn't say his first word until 17 months (apple) and definitely said fewer words than yours at 18 months. We were told it didn't count as a delay until they'd not said a single word at 2 years old. In case it helps, my eldest is a complete chatterbox now with quite an advanced vocabulary. Lots of kids are less chatty at nursery and all mostly play by themselves mostly at that age - they're still in parralell play.