J.S.
Have you tried doing your own version of "play therapy" with him? I read a book calling "raising a secure child" that talked about doing this. Basically, set aside about an hour of time to play with your son -- just the two of you, make sure you have someone else to take care of your daughter. Have playthings that he can use to act out how he feels like a house with people (or a castle or fort or whatever if that's what he has), building blocks, etc -- open ended toys. Let him lead the playing and just comment without judegment on whatever he does such as "That guy is hitting the other guy." Avoid questions. The only rules are that if he hurts you or if he damages the room the play time is over. After 45 min to an hour it's over. The purpose is to give him a safe place to work out his feelings. Eventually you will be acting as a coach to help him get things out and learn how to use words to work through things or even to understand what he feels. You can look up "play therapy" online to get ideas, too. I'm just telling you what I remember.
I actually did this with my son after my daughter was born. He was 2 and got really aggressive with her and with me. So, I started doing this form of play with him every day during her afternoon naps. It really helped our relationship.
That's all I could think of (other than professional help), Good luck to you.