I have a quiet child, and a child with SM and autism
The quiet child is a man of few words, but speaks freely when he does have something to say. He can order food in a cafe, go into a shop and get stuff, can ask his teachers if he is not sure about something. He can answer thoughtfully if asked an open ended question, will have a debate about something, joins in on family events happily, but he isn't going to come and seek you out for a chat. He talks up a storm with his small peer group of friends and is going out with them appropriately to the park etc.
My child with autism and SM was always hard to "get to answer". Even as a toddler he shut down on questions which he saw as exposing. I remember having to use ridiculousness to get him to engage "What did you have for lunch?" No response "Was it pizza?" No response "I know what you had, I asked your teacher, it was dinosaur on toast" no, don't be silly, it was fish and chips!. He has a consistent pattern of speaking - spoke to family, albeit preferred to lead the chat, use banter, speak about his preferred topics - in school could answer clear questions with non ambiguous answers, such as "what is 6x8?", And yes/no questions, but unable to initiate any communication, couldn't speak at all at doctor, cafe, or shops.
It's the shut down that marks out the difference for me. My SM son you can see his expression close when he gets asked something or spoken to by a new person, you can see the anxiety appear and his voice shut down - and he's an adult now.
I did use to use the a) and b) method of communication a lot with my autistic/SM son, and also sort of flow chart responding. Eg once he was obviously upset and couldn't tell me why. I had to go through "is it about school, or home" "is it about work in school, friends in school, or something else?" "Is it something your friends have done, or something they have not done?". It's like he couldn't find the words to express himself without me structuring a way in. Then eventually you'd get a trigger and the story would pour out.
Does it seem like the expectation to speak is sometimes a problem for him? This is the essence of SM.