Another idea which may be of use to you is (if you haven't already) run through some different scenarios with your child using the trigger that causes him to lash out.
Eg if your child lashes out when he's frustrated then gradually expose him to situations that are frustrating eg waiting for your turn, waiting in a restaurant. Model for him the correct behaviour whilst acknowledging his feelings. Then praise him for doing the right thing.
If your child lashes out due to anger then watch for moments at home when he is angry but does not hit then name the behaviour and praise him to how he's handled his feelings.
If your child lashes out because he's lost a game then play a game with him eg monopoly where you lose. Model for him appropriate behaviour if you lose. Talk about being a good sport. Then have a turn where he loses and comment on his behaviour eg I can see it's really hard for you because you really wanted to win that Monopoly game. But what's really important here is being a good loser. I can see you're being a good loser because....
You could reinforce the positive comments with eg marbles so every time you see him not lash out when he's angry he earns praise and marbles. A full marble jar earns him a day trip to the beach, an hour alone with his mum, a new book - or whatever works for him.
Using the above idea then you're attempting to mould his behaviour. Simply telling him not to do something is not enough.
Also, I think it could be useful for you/your wife to keep a log of incidents. What you need to do is to aim for progress not perfection eg child hits 4 days a week, this goes down to twice a week, once a week, once a fortnight, once a term. This is a really useful tool both for yourself and others to see that things are moving forward.
I think for lots of posters your post stirred up very confronting emotions but I can see that you are a concerned parent who wants to take on board practical advice.
Try to focus on positive reinforcement and make a huge effort to catch your child being good. Best of luck with it all.