M.I.
Hi K.,
I'm in a similar situation with an 18 month old and a 3 month old (and preemies to boot!). Big brother was not too happy when we brought little brother home from the NICU either. At first he would have nothing to do with me or his brother and even blew raspberries at him! Slowly he warmed up to both of us and now likes to give him 'kisses' although we really have to watch.
So far when he hits or pokes his little bro we tell him "nice hands" and show him how to touch nicely. It can become tedious and it's important to make sure that the baby is not left within his reach unless we are right there. I was afraid it would be a battle but it does seem to be getting better and big brother has become a great 'helper'. Good luck to you!