Hi there,
My son doesn't have autism, but I have been a behavior therapist for a few years (plus my son has aggressive behaviors resulting from a brain injury).
I know how stressed you must be and I can empathize. Here are a few things that I learned with my son and from my training/working one on one with children diagnosed with autism. Please forgive me if you already know this.
Behavior always has a function. Typically functions are either attention seeking, self stimulation, access to desired items or activities, or escape from aversive stimuli. Once one determines the function of a behavior an effective intervention for that behavior can be developed.
Try classifying your child's behavior first (and take your time as I know you're overwhelmed by the behavior in itself. I wouldn't want you to become overwhelmed by trying to find the function of that behavior.) Make a list of each behavior he demonstrates, ie: tantrum, aggression, non-compliance, repetitive behaviors. Then write out the definition of for each behavior ie: aggression = hitting with open/closed fists, biting, throwing objects in the direction of others, slapping, pinching, screaming into others ears/faces, etc. Once you've done that for your kiddo, think about the times he engages in these behaviors and what happened directly before the behavior, ie: did you say it was time to turn off the T.V.?, ask him to come to you?, go somewhere he hates like big stores?, ask him to wait for your attention or for an item? You can then determine what the function is; does he want access, escape, attention, or self stimulation. Take it one behavior at a time.
To help manage behaviors, positive reinforcement is necessary. I won't lie and say that it's easy and takes no time at all. It's hard at first but it pays off. Find out what his highest preferred item or activity is, this is called a 'reinforcer' and kids with autism usually have about 1-2 high interest items or activities, even food.
When you know you have to do something important with him or go somewhere that's usually stressful, deprive him of that reinforcer for awhile beforehand then give it to him during that important time only. You can create token charts where he earns a token doing super simple tasks, like simply acknowledging you've called his name. Once he gets a token, reward him with his reinforcer. As time goes on, less reinforcement will be needed. Ask a behavior analyst for tips on managing behavior, if there is any in your area.
There is so much more that goes into this - the model I have referred to above is called Applied Behavior Analysis or ABA, and it's effective when used effectively.
I truly hope this helps! If I haven't and have further stressed you out, please forgive me.