1) how is his Teachers Handling it? Perhaps, you need to keep it all consistent... and do the same thing at home too, If, the Teacher's technique "works" at school.
**OR, is something at school causing problems for him? Is other kids giving him problems? Is he being bullied? Is he being treated fairly by the Teachers? ANY problems at school that is "TRIGGERING" this in him???
** Is it ONLY at school that he behaves this way?
2) A new child, is a BIG change for an elder child. ANY "regression" or behavioral change (for better or for worse), is probably triggered by the new baby, for your son.
His "regression" and physical lashing out... in any child, is a SYMPTOM of stress/frustration in a child.
3) Getting used to a new baby, takes time, for the existing child... the "hierarchy" in the family has changed, many things has changed, for the child. Your son "had a baby" too, not just Mommy. Yes, your baby is now 11 months old, but your boy is still acting out... he has not adjusted yet, which he is obviously showing.
For me, my daughter was just about 4 years old when I had my 2nd baby. The whole time prior and then once the baby came home, my Hubby and I explained to her that: "tell us anytime you feel unhappy or frustrated... having a baby will be different, but tell us anytime you have a hard time..." or, "we don't expect you to be grown-up, you are a child yourself, and you will always be our FIRST baby...so tell Mommy or Daddy whenever you need help with your feelings or if you feel upset..."
The thing is, although my daughter LOVES her little brother to bits... there were times she just felt SO frustrated and IRKED, by the baby... and she "felt" as though she HAD TO BE "perfect" because she was now the eldest child. So... we observed her and KNEW that about her... so we always made sure we talked with her and told her that by no means, does she have to be that way because we do not 'expect' her to be perfect or any different just because she is the "big sister" now. Nor did we 'punish' her for frustrations she had... we just 'consoled' her and help her through her feelings... we always talked with her about it. So, we didn't focus on 'punishment' or telling her she was 'not acceptable' but rather, we used the technique of "empathizing" with her, showed her that we understood where SHE is coming from... then explained that there are other ways to show frustrations... that we are a FAMILY and we are a TEAM... and we ALL love her very much...and understand that this is not easy for her either. I even told her "It's not easy for Mommy either... a baby cries, a baby can't do things by themselves, I wake up at night, I get frustrated too... but Mommy still takes care of both of you... because I love you.... don't worry."
There were a few times she resented her little brother... but is was not her 'fault.' She was just needing us.
Remember, there is nothing wrong with a child wanting more attention. Its normal, once another baby comes into the picture.
You want to teach your eldest child, HOW to be a sibling now. They do NOT automatically know how. AND, don't expect the eldest child to be "perfect" nor use them as an "example" for the baby... it will always be resented. No child likes being compared.
Teach him how he can help and love his baby sister.... and that even Mommy has good days and bad days... but that "we all love each other.... no matter what."
My daughter would ask me "Do you still love me, you spend so much time with brother...." And I would pick her up, hold her in my lap and tell her "YES, I always love you no matter what... you are my FIRST baby, and you are special for that, don't worry... Mommy always loves you even if you are grumpy or not... Mommy isn't perfect either...." And telling her this would always maker her feel better, and that even her own Mommy had 'grumpy' days too. So she felt understood. I even told her a few times " If Mommy forgets to kiss you when you wake up, you be sure to tell me... I don't mean to forget or make you upset.. I know it means a lot to you... Mommy is so busy, but you can remind me anytime of what you need...." and she liked being able to "remind" me of things to and that I understood.
Let your son adjust... still. Don't punish him for valid feelings that he has... let him talk to you about it candidly without judgment. Children, girls or boys, NEED to vent and be able to tell their Parents how they feel, no matter what and even if they are grumpy. They need to feel "accepted" in light of the new baby.
This is what helped our daughter...
All the best,
Susan