First, do you spank or potch? If so, the hitting could be a behavior he learnt from you (I’m not saying it is, just posing the question).
Also, its important to teach him to deal with his feelings. Start helping him label his feelings (“I get frustrated when older brother gets to do things first” or “I get upset when younger brother takes my toy” or things like that). The more he can verbalize his frustrations, the less he will tend towards non-verbal communication (which hitting is).
Don’t discount the playgroup – my friend’s son just switched and a lot of his negative behavior went away. Does he like to go to playgroup? Does he want to avoid it?
I definitely second the alone time. Its really important for your kids to feel like they can be heard as individuals. That goes for all your kids, but this one especially.