YOu don't mention what you have tried.
First I would say that he should know that he is NEVER allowed to strike anyone, especially an adult. He should suffer consequences that mean something to him immediately.
Now do you ALWAYS exact a punishment for bad behavior? Do you ALWAYS praise him when he does the right thing?
Do you tell him you love him when he's bad and have to punish him? Do you insist that he follow the rules? He's old enough to understand.
If he pitches a fit when you tell him no then he should suffer immediate consequences. Put him in his bed/crib and tell him when he stops throwing a fit, he may come out. If he won't stay put, then I would suggest a spanking on his bottom. Then put him in his bed/crib again. IF you have to, then physically make him stay in timeout. Stand there and calmly tell him he has to sit in timeout until he stops with the fit.
Don't yell. Stay calm. Walk away if you find yourself losing your temper. But make him do what you tell him.
It may take a few weeks for him to get it, but he will get it if you are consistent.
Every child is different and you have to figure out what he will not like as far as consequences.
You may have to stop what you are doing to make him mind. You may have to leave a store or make him come inside when he's playing. It may take you 30 minutes to get him to calm down from a fit, but when he does praise him for making the right decision.
He will be much more work than your oldest. My second child is way more work than my oldest.
Here are a few things that have worked with her:
1.) I always make her make eye contact with me when I'm telling her to do something or I'm reprimanding her about something.
2.) I don't let anything slide.
3.) If I have to physically hold her hand and make her do something I will.
4.) I don't beg or ask repeatedly. I try to remember to tell her once and then I might count to three. She knows that if I get to three she's in trouble.
5.) I praise her when she does something right. Like she plays nice with her sister or does what I've told her the first time.
6.) If I have to discipline her, then I always tell her that I love her and want her to behave. I will hug her and hold her after she's calmed down if she wants a hug. I always remind her that when I discipline her it is because she made a bad choice and decided to do something bad. That we always suffer when we make bad choices.