I can relate to your situation. Thank you for sharing your story, it makes me for a little better knowing I'm not alone in this.
I have 4 boys, my 3.5 year old sounds similar to your son. His younger brother age 20 months is often on the receiving end. Thankfully there isnt much biting, but there is a lot of hitting, poking and pushing. Most of the time they hardly play with each other. My 20 month old will try but that usually upsets my 3.5 year old and he'll just go somewhere else.
It never occurred to me that my 3.5 year old might need more one on one time with me. It is nice hearing from other moms and how they deal with certain things.
:)