I don't think there is much you can do during the tantrum, except try to keep him and you safe.
There are two holds you can try to keep yourselves safe until he is able to be calm.
One is you sit down, and sit him in your lap, facing outwards. Place your left leg over his left thigh and your right leg over his right thigh. This way, when he kicks, he has to lift your leg, and this diffuses some of the force. Cross his arms in front of your chest, and hold his forearms, in a bear hug. Don't hold him at the joints or apply pressure to any joints (wrist, elbow, or knees). Don't pull his arms so tightly that his breathing may be restricted. Now, you just have to watch out for head butts. You can either put a pillow between his head and your chest, or lean slightly to the side, so it doesn't hurt as much.
The other type hold is lay him down, face up (never face down). Kneel over him. Put your knees on the outside of his legs. Put your calves over each of his legs and support your legs on your feet/toes. Don't rest your legs on top of his; you form a kind of bridge over his legs with your legs. Spread his arms out, and putting your weight on your fingers, lock his forearms between your fingers and your thumb. Aqain, you don't put your weight on his arms, you form a bridge with your hands.
Then basically you just wait. Words won't have much effect; he won't be able to process what you are saying. Take long, slow, deep breaths. You could sing a lullabye, or recite poetry, or tell a soothing story. When he is calm for 60 seconds, let him up. If he can't control himself again and hits, hold him again if needed.
Passersby might look at you funny and you might feel odd. People will probably ask if you need help. Responding calmly that you have it under control will help your son realize you have it under control and he is safe.
All the work in decreasing tantrums happens before and after the tantrum. Nothing is really learned by the child during the tantrum. You might try to look at what happens before, exactly. Is it always in a certain place, like a store? At the same time of day? How long since he has eaten? Does he want something that he can't have (like demanding a toy from the store)? Is it for attention? Is it to escape from the place? Is he overstimulated? It won't do any good to ask him why he is threw a fit - he probably doesn't know and will tell you something, but it won't be accurate. You have to be the detective.
Once you have an idea what might be causing the tantrums, you can set up "practice" sessions, knowing you might have to leave suddenly and having an escape plan. If its stores, you might try talking with him beforehand, explaining how you would like him to act, and get him to engage with you about proper "store" behavior. Start out by just going in the front door, staying maybe 30 seconds, and walking back out. Or whatever you think is a good idea, increasing the time spent in the store, etc. If you use praise, give lots of praise for successes, etc. If you are okay with rewards or reinforcement, you might occasionally give him something on the way home for successfully being in the store (but don't promise him something ahead of time - e.g. "If you can be calm and walk next to Mommy in the store, you can buy a toy" because then he will expect something everytime - it needs to be a surprise).
Hang in there... it will get better, even if you decide to do nothing differently. These are only suggestions.
right now
: Mine have done this, too, and I usually just pop them in a cart or something and let them do it, pretending I don't notice. They quit pretty quickly. I DID get some nasty looks though.
