Have you tried a behaviour chart?
Maybe he is feeling a little left out as he has younger baby in the house.
I have three ranging from 8 to baby.
We have a chart on the wall for the older two. We have job lists so that they can start to take some responsibility for earning pocket money and also have a space at the bottom which reads "tv and playstation". If they end up with 5 naughty spots in that box then they lose their priviledge of tv/ps for a week.
They then realise that there are consequences to their actions - bear with it it really works.
We have things on the chart such as "respect, attitude, no shouting screaming aruguing etc." then we have things such as "pack school bag, empty lunch boxes, tidy bedrooms" they then get a dot in each of those as and when they are done, they can then see progress in good behaviour.
Their favourite is the "merit" points, if they get 5 merit points they then get a suprise, something they might like such as a new book etc.
We keep sweetie day to a Friday, not allowed them through the rest of the week, if they are naughty(as in built enough naughty points on the chart) they lose sweetie day or pocket money depending on severity.
It will only take a couple of weeks for the child to realise that they are causing the pain to themselves with loss of things they enjoy.
Try it it works!!!