C.H.
Oh how he is approaching this all wrong!
So, open the conversation and tell him nicely how you feel and what impressions you are getting. Does he not feel confident in you? Does he always ask his mother everything and take all her advice? Does he feel confident in researching tehse things himself?
Let him know that it really isn't a good idea for him to tell you that his "mother thinks this or that is a good idea." Makes him out to be the boy who can't think and speak for himself. Even if she does offer a suggestion to him, he should research it first. If it still sounds reasonable, he could say something like this: "I noticed little Susie doing ___. This book and this website I was reading said that kids normally start to do this at this stage. Do you think we should start that now or is there a reason not to. If he wants to be involved in those things, which is good, he should or you could get him a good childhood development book and a website for spontaneous medical questions so he is coming from a somewhat, educated place.
Good luck. he probably isn't confident of his own opinion or lack of research so he thinks it puts more weight to back it with "Mom said." Let him know that doesn't give it weight, it gives y ou frustration that he doesn't approach it as an independant adult.
My daughter consults me often but she learned not to tell her husband my comments. She invites him to research it, ask his father or mentors. He doesn't like to ask others as he thinks he should know so he just guesses and tries to sound confident. She lost respect for him doing that so she's leading him another direction and she's learned to leave my name out of it. I also underscore to her how she could have found that information herself. I won't be here forever and I'm a good reader, researcher.