S.Y.
I have a few suggestions:
Get "Signing Times" DVDs from your local library (or buy them on Ebay). My dd knows over 20 signs and it GREATLY reduces tantrums and frustration. It will take a few weeks of daily watching and you will have to reinforce the signs. It's fun to watch and do with your child.
Get "Positive Discipline" by Jane Nelsen. GREAT book...deals with this problem.
Do what I call the "hug and hold" method. Next time he hits, calmly put him on your lap facing out and gently hold his arms next to his body..."hands aren't for hitting"...hold him there gently for a bit. He won't like it. After a few minutes, release him, and show him how to ask for something (sign "please"). He will do it again, so you passively GENTLY restrain him again. It will take a few times but he will get the message. You can check some of my past posts to see me expand on this. Good luck....:)