at this age of 9, they are pre-teens. Your son is a "tween". Tweens are from 9-13 years old. Some kids this age start to change biologically AND emotionally... many hormones, developmental changes etc. It must be a doozy for him. He is 9.... and a child. His world is gonna change... and each age juncture brings with it different emotional/developmental issues and complexities.
So he is dealing with that, and having a baby, and his parents changing with respect to that and so many things a child's Imagination and feelings encompasses.
He's feeling insecure and even not knowing how to feel or cope. If he senses that "he" irritates everyone... well, he may regress further. Mainly let him know he's okay and its okay to feel uneasy, and that he can talk about it.
"Regression" in a child is a symptom of stress. Or that they don't know how to cope with something.
Give him time. Not base it on his "age"... nor have expectations about his behavior based on his age solely.
Maybe, and some kids are this way... they feel stressed about being an "eldest" child...and all the expectations upon them. It is a big thing to carry on their little shoulders and they cannot always handle it. "Expectations" of a child (by the parents), does not equal "readiness" of the child.
Try not expecting him to be a certain way, just because of his age.
He is probably a velcro with his Dad... because his dad is his "soft place to fall".... and all children need this, and at times like this. Its good, that he bonds with his dad and feels affectionate with him and looks up to him... for "problems" and what not. Nurture that. It is important for a boy... and since he will be entering into teen-hood. Once he is a "teen" and a pre-teen... you want to be SURE he continues to come to his dad or you... for anything, problems, questions, anything. Otherwise... if he goes the opposite direction & shuts you out and his Dad, you will have a BIGGER problem and more rebellion and him not even opening up to you both nor telling you about what is going on in his life etc.
All the best,
Susan