Have just noticed in another post, that you have a 9 month old too. Could your son's misbehaviour be related to the fact that he is feeling a little 'left out' with his brother around??
You probably DO try to include DS 1, but remember too, all the cute things your baby is doing at this time, and all the little comments ...i.e.
"Awww Look how he is doing ...."
"Aren't you such a clever boy to be ..." "Wow You ate a whole ..."
... well that's me anyway [LOL].
My point is that all the cute things that we comment on could be making DS 1 feel a little inadequate - and he may need to be reassured more than ever.
I used to always try and include the older sibling, if in earshot!, in any achievements of the younger sib. i.e. "Look how she is crawling so fast. That is because she is trying to go as fast as you - she real wants to be able to walk like you."
"She said ...!! That's because she has heard you say that. You have been teaching her to talk just by talking to her!" etc
I don't seem to be coming up with great examples - but you get my drift, anyway
If you give us more specifics I am sure it will be easier to offer something more concrete!