My daughter does this...and it's not defiant behavior he's trying to build a dialogue about the baby...probably from stuff he picks up from other conversations or may even look it up himself. He feels he's being helpful and included this way...yes it may seem defiant and definitely annoying but it's pretty typical autistic behaviour. For example my daughter who is 9, told a lady in a store directly to her face that she was wearing too much perfume. She wasnt being mean or mouthy she thought she was solving a problem and helping the lady out. She's very literal, and blunt. She'll get stuck on certain things when she feels she isny being listened to or dismissed too. The other part of the know it all attitude is he's 11...preteens really think they know it all lol. I expand the dialogue to get her to lose interest. Example he says you need to change the diaper...I would say, " Yes maybe she does need a change...but I bet you cant guess what size diaper she wears...do you know how diaper sizes are determined...do you wanna know what brand she wears..." By then my daughter would have already started tuning me out :). Also change the subject become interested in things he's interested in and talk about them. Parenting an autistic kid means you have to completely change the way you do everything...I suggest you read up on it. The typical "I'm the adult your the child thing" has never worked for me.

Perhaps asking for his help immediately after he notes what is needed to further his understanding of the different situations. He truly is trying to make sense of the baby which doesn't make sense to him at all.

What a hard situation! Personally if it were my child (which yes my 7 year old tried to do the same things with me in reguards to his younger siblings) when he starts in on me I hold up my finger and interrupt him saying “who’s the boss?” Lol he says “you” and then I say “exactly! I’m the boss, I do the telling!”. You could try that maybe, then play it off as a joke if it goes badly just like he does you! Good luck!