Not read the rest of the thread, but it seems to me that you need to make your understand your displeasure and underlining how much you disapprove of trying to get one;s way by violent.
For this reason, I would very heavily caution against hitting him back, as it would give all the wrong signals. I do not knwo what you normally do to punish him- naughty corner? Naughty stair? time out in room for x minutes? Withdrawal of privileges?
It seems to me that timeout in his room for x number of minutes, letting him out only if he apologises explicitly for what he did wrong (ie- I am sorry for hitting you Mummy) would be the best reaction, and withdrawing things if he refuses to apologise (ie accept what he has done wrong) is by far the best raction. He is not too young to understand any of this.
Also, his dad has to underline to him how wrong it is to hit. It cannot be OKK for one person, but not for another, as any child seeing that violence works where other things do not will almost invariably gravitate to violence to get things done. It seems to me that shocking him by suddenly withdrawing attention from him might be just as effective as any kind of smacking response. As would ostentatiously putting a toy of his into a black plastic binbag and putting out for the bin men.
The tantrums, I'll wager, are his first line of getting what he wants, followed by anger, followed by hitting. He has to start to use more words and less action to negotiate his way. By refusing ot be sucked into his tantrum, you will be helping him. He is not 2, he is 6. He needs to learn to be more social; oral negotiation is the civilised way of obtaining this.
Next issue is why your husband asked him to get off him and was ignored 10 times, which led to that situation you described. Maybe he is feeling a little neglected emotionally by your husband ans was seeking attention? Perhaps he needs some organised father-son activities to defuse some of that very real need for physical horsing around, and male bonding experiences? Does your local judo club have a parent and child session? That could be an excellent opportunity for them both to do something fun together. You may need to organise it for them! (I peak from experience...)
Good luck.