I feel for you my friend! you have a strong will child who may also have some kind of disorder. My now 17 yr old daughter was like that - and we had some very trying times. She was diagnosed with a disorder - which was devastating at the time - but it explained so very much.
When she had her horrible tantrums (that would go on and on and on) I would direct her to a spot in the middle of a room where she couldn't hurt herself, and I'd do somehting nearby that needed to be done (wash dishes, vacuum, dust, clean out coffee table storage) so I could keep an eye on her but "ignore" her as well. I wouldn't get emotional (I did the first few times when I had no idea what was going on) but very deadpan I'd give her direction (don't hit into the furniture, watch out for your head, etc.) but continue doing what I was doing. If we were in a public place I'd have to pick her up, and leave. Twice I had to tell the grocery store manager that I left a wagon in "aisle 7", I'm so sorry but I have to take this child home. They were wonderfully understanding.
Put your music or TV on louder, vacuum, do something to distract yourself becuase you will surely begin to doubt your abilities to parent this child. I was certain that I was a failure....
Once you can find out if there's a a diagnosis that covers yoru child, read as much as you can about it - then begin to follw that guidance. When my daughter was 14 I had to tell her that I would outlast her, if I had to sleep in her bed to keep her safe I would, if it meant that I had to quit my job to watch over her and keep her in line I would. If I had to take out a loan to get her the help she needed I would becuase she was more valuable to me than any car, new kitchen or vacation - and if I would take out loan for those I'd do the same for her.
Try to figure out if there are triggers - food, times of day, transitionary times. My friend's child was diagnosed on the Asbergers spectrum. when she removed gluten from his diet he improved within days. As they kept it off the diet he eventually came off the diagnosis and he's amazingly well. My 17 yr old is on her way to becoming a wonderful young lady. Getting a name for her disorder and learning how to address is has made all the difference. She's a good student, she works PT, she volunteers a few times a week and just came back from a week in central america holding camps for really, really poor kids and we're planning for college in the Fall. There is great hope for these kids who make us crazy - I think they end up being the leader, movers and shakers of hte world.
When you're ready to lose your mind remember if you can't find a way to control yourself then how can this young child figure out how to control himself? Dont take it personally, don't get mad at your husband becuase you're always the one in the middle of these event, think about how to show love to your child who is wildly unhappy becuase he doesn't know how to control himself. When the tantrum is done hold him in your arms and tell him how much you love him and that next time he gets really upset he should tell you and you'll help him deal with it. Eventually he'll get it, eventually it will begin to stick.
finally, when you do lost it and you've now upset with youself - give yourself a break. Forgive yourself. Ask you child to forgive you - hold eachother, include your older son, and move on. Pray for God's grace to cover your errors - we are all human and have all been there.