Diagnosis in toddler

I am the grandmother to twins who were born 4 weeks premature. They are now 15 months old and whilst one is achieving her milestones the other is not and displays (in my opinion) autistic behaviour. Their mother is unwilling to accept that he is behind ... just chilled. They have been today for their development check (12 months) albeit late due to covid, and the health visitor has apparently deemed them both to be perfectly okay. I am frustrated in the knowledge that my grandson most certainly is on the autistic scale and yet getting nowhere.

Any advice would be greatly appreciated