Why can't you discuss with your daughter?
Many things I would say wait and see, but autism isn't something that there is ever a good reason to leave it and see. It's always a bad idea.
My two both have autism. Early intervention is important.
I always say it, but an assessment does not give an NT child autism but it always helps a child with ASD. so there is never a reason to not assess. nobody has any reason to give a child a dx of a condition that they do not have.
If you can't talk to your daughter, can you talk to your granddaughter?
I'd talk to them directly if needed.
Or have a word with the health visitor.
If you are wrong, then you are wrong. You are raising concerns, if it turns out he doesn't, then that will be the end of it. They may roll their eyes at you but you can cope with that. It doesn't matter.
If they wait until he reaches school age and it turns out he is on the spectrum, then he will have missed out on a great deal of help and support.