Is he getting frustrated because he can't communicate with you? If not, then I think he is talking just as much as he needs to.

If he is the younger child, then maybe the older kids are "talking" for him and he doesn't need to IYKWIM. You might need to ask them to stop. My sister had to do this with her two older children. They talked for their sister so much that she barely talked until she was 2 and a half because someone else took care of it for her.

Were you concerned before you SIL said anything? If yes, then take him to the GP but if not, then I suggest give him more time. If not much has changed in 6 months, then get him checked out just to be on the safe side.