I think the above is a really good question. What triggers the tantrums? Looking for the root cause of a behavior issue can help you get to the bottom of resolving it.
How is his speech, for example? Does he trip easily? Does he have a hard time self soothing? Is he over tired?
When you have a child that is explosive, it is an understatement to say he needs to learn appropriate ways of communicating. But in a nutshell, that is what needs to happen. First, at home it must be calm. Parents can't yell or spank as it is hard to ask a child to control themselves if the parents arent'. (not saying that this is going on but just making a general comment). I'd do some modeling/role playing where you act out being frustrated. He won't know you are acting-------- but be exagerated so that he can actually see some behavioral alternatives in action. It will plant some seeds of ideas of how he can act if you when upset, mad or frustrated.
I'd go to the library and check out books on emotions and read them to him. My kids talked about anger as a "tornado" and would tell me a storm cloud was blowing in as they were getting upset. Kid language for what is going on inside so that he can become aware of it himself.
There is also a series of books with one called "Hands are not for hitting". Great book and it is awesome for driving home that message for kids.
I'd try to give this boy choices whenever possible. You control the choices so you are still in charge------- but he is more willing to comply if he has had some say himself in what is happening.
I'd make sure he is getting enough physical activity. He needs to be running around at parks and playing hard. This has a direct affect on behavior.
"SOS for Parents" is a book that discusses how to use the time out system and it is good. I personally like "natural consequences". If you throw a toy, you lose the toy. If you are yelling/crying/whining, I will say I can't talk to you until you calm down because I can't understand you. After a while, they see that their own actions result in either good or bad things for them.
Also, remember that kids will seek attention anyway they can. If they don't get enough positive, they'll get it through negative behavior. So, make sure you make a HUGE deal when he does as you would like and praise him like crazy. Make him your helper as well.
goodluck
What is happening at home, any new siblings ,a house move , anything happened in the family may have upset him .What are the trigger points , what happens prior to the tantrums ,how do you and his Dad react to it ..