While I would agree that parents should move forward from sippy cups in general, when you are managing a child with special needs and specifically oral sensory needs, you need to look at the bigger picture and what the child needs to cope with the sensory world around them. Blindly following guidelines because they are there rather than taking into account the specific sensory diet needs of a child doesn’t help the child, in my opinion.

I was surprised that you are in the field of early childhood, but recommend use of sippy cups past 18 mos.
The enclosed reference sites some good reasons why parents should be quicker to avoid or quickly transition children to open cups.

If you live with your grandchild full time and cannot understand what he is saying, then yes, it’s an indication that he should be evaluated by your school district.

Actually, your child should be talking around age 2. But if they reach age 3 and are not very verbal, it’s time to get an assessment. It was a red flag for us and we got him the help he needed.