Just read your story and it sounds like you are still new to Autism, I just want to tell you to hold your head up because you do have a great kid and he will do great things. Have confidence in yourself you are a great mom and I can tell that from the love and how you advocate for him already.
As for tantrums it is hard to say what is best. I found tracking the triggers, time, activities, sleep, know pain, BM , what helps keep him cal, etc. has helped me identify the causes. Knowing the triggers and causes for decreased ability to cope has help me learn to prevent some or given me ways to calm him. Sam has a bag of webkins that we take to all apts, church, etc. this way he has something that he likes with him and petting them, hugging them and fixing their hair helps him to stay calm when he startles to get upset and to help him once he starts calming. Sammy (7) also will calm if I hold him and rock him, some times I have to sit on the floor and cradle him like a baby and rock him and sometime I can stand and rock him. During full blown tantrums I haven't Lund much but to keep everyone safe which sometimes means using holding type of restraints bear hugs or others that I have learned with the 13and11yr old.when hitting, That is where you can't just move away, I hold their wrist just tight e nought that they can't pull away and tell them "no hitting" then if they are trying to pull away I say " you are having a hard time controlling your hands I'm going to help you keep safe until you are calm enough to control you body yourself" I don't wait till totally calm to lt go just till they start calming ( or they become too tired to continue). We have different sensory stuff around the house and 2 of them have sensory boxes so when we see them starting to escalate we prompt them to try their blankets, their boxes, go to their rooms to calm, or with Sammy to hid under his blankets or in his fort. Sammy has a play fort that only he is allowed in that helps him to get away from everything while keeping him safe. He will hid anywhere and he also runs so it is hard because if we don't let him hid he runs but I want to see him to know hat he is safe. We also sometimes avoid triggers until they learn to deal with them. My 11 couldn't go to any store with me or over 2 yes because he thought he had to have something every time we went into one. I had one episode where he layed on the floor at the end of two check out lanes at toysrus for 45 min screaming because I wouldn't buy him a $75 toy and he only got the original toy we went for. Good luck.

This is often hard, but I agree you need to see if you can figure out the triggers that are setting him off, and try to avoid those. I will say that sometimes there are no apparent trigers, so you also need to try to employ some other tactics such as diversion with something he likes, which will not always work either.

Sometimes it is just best to let them get through their tantrum without doing anything, because no matter what you try to do to calm them you are just making things worse. The only exception to this is of course if they are hurting themselves, then you need to step in. If you have not done it already you may want to consider some ABA treatment for him because that helps as well.