I'm not sure if I'll get flack for this or not but I have spanked him (never hard enough to leave marks, he laughs when i do it) and I'm not really sure what else I can do.
As Susie said, we've probably all or most of us tried this. I did too. I can remember times when I spanked, in anger, and felt I had no alternatives left. Susie is right, especially with autistic kids it actually is a bad idea because they then learn that if you want something from someone, you have to hit them. These kids learn by imitation.
Now, I've noted others reckon you're not getting enough help. That may or may not be the case - I can't say. But I can tell you, we came a long way by working it out for ourselves when we didn't have any other support. A book I suggest you try to read (I know, it's difficult - I actually didn't finish the book, I took ages and only read a paragraph or two at bedtime before I fell asleep!) is "Son Rise". By the time I read it, I had already done a lot of the same sort of stuff. Basically, it's what I call instinctive parenting. As Busywend just said, sometimes you need permission to step away from the norm. I learned that it was OK to break the rules. Some kids do learn to walk before they learn to crawl.
In summary - go back to basics with your child. Spend some time just observing him. Take notes - pretend you are a professional, assessing him. Observe, think, analyse, guess if you have to. Then test - what is it you think he is doing?
You start where the child is and meet him there. In "Son Rise" the author describes how his infant son would spin plates. Constantly. Interrupting him would lead to raging and tantrums. So the dad sat beside his son, on the floor, and got his own plate and began to spin it. He did whatever his son did. And never forget - this is the starting point, not the end point. Your child is different, his brain works differently and HE is trying to study himself in relation to the world; he is trying to find his own way, and if you join him you are saying, "It's OK to be who you are. I am here beside you, I will help you."
Especially in the younger autistic person, the frustration level is high. Their only method of communication is the tantrum. It's your job as parent, to read their mind! The child cannot understand why you do not understand what he or she is thinking. So you need to establish communication with him, find ways to help him. Don't try to control him; keep him safe, but otherwise step back. Discipline needs to be handled differently and frankly, you may need to simply keep him safe by using removal and distraction/deflection. Assist with communication - Compics are commonly used. But remember symbolic communication is a challenging concept for someone who has a communication disorder.
In our case, Compics were of limited use, because difficult child 3 has hyperlexia (which as far as I can understand it, is a subset of high-functioning autism and Asperger's). difficult child 3 was obsessed with numbers and letters. Interestingly, a good friend of his (a few years younger) is also hyperlexic and had language delay. We did the same thing - we labelled everything in the house. So he's only 3 - so what? We had a two year old who was not talking, unless it was a word he could read. THEN he would begin to use the word. I also made little books for him - I got a sheet of paper out of the printer and folded it in half. Turned it, folded it in half again. And again. Stapled down one side. Taped over the staples. Then I got scissors and cit the folds so I had a little book with 16 pages. I then used this to draw a little picture and write the word on it. I chose words he needed (such as his own name; my face and my name; "stop"; "go"; "drink"; "bath" - whatever you need. We also had to practice the words, over and over. We did it like a game. Do everything like a game. All of life is a game, and games are how kids learn. Play is serious educational stuff.
Forget about 'age appropriate" - if your child can do stuff that others can't, encourage him. Don't try to ever put him into a pigeonhole. I had a baby who could use a computer (we scavenged one off the local street clean-up - figured if the baby broke it, it wouldn't matter) and installed a lot of educational games on it. I would sit with him and play the games with him, or help him do them. A lot of the games were things like mazes, or match the letter on the screen with one on the keyboard. He also would sit on my lap when I played piano, but I had to let him take over - forget about Rachmaninov when the baby wants to play. I let him, and he did not bash at the keys like other babies do; he touched them gently, and then tried to play two notes and I could see him bending in close to listen. He would put his head hard against the piano, or the stereo speakers, to listen to music and feel the vibrations. A cousin of mine was reported to have done this as a baby also; he turned out to be a musical genius, a modern-day Mozart who later became a world-famous computer (now dead - he was in his 60s when he died). My cousin was a bit socially inept and completely obsessed with music. Interestingly, in the late 70s and 80s he did a lot of work with autistic kids using music as therapy.
Back to your son - what I'm saying is, follow his lead.
If he were a puppy and you were the dog whisperer, what would you do? Find something that is an immediate reward and stock up on it. If it is praise, then use it. If you need something more concrete (such as a pocket full of Cheerios) then use it. Your son is the puppy and you are Cesar Milan. That is your starting point and going back to basics can make a huge difference. You can never turn him into a normal person. But you know what? Being "normal" may turn out to be a come-down for him. These kids can end up being amazing, especially in how fast they can learn what they need to.
It can be a rough ride, it is a ride full of surprises, and it can also be very rewarding. But you will never be bored!
Your son is an individual, some of my suggestions probably won't work. But along the way you will find your own, and work stuff out for yourself. That is your aim. Your son will benefit. So will you.
More reading (sorry) - "The Explosive Child" by Ross Greene. Check out the sticky in the Early Childhood forum.
Anything by Temple Grandin - she was once like your child. She is now an Associate Professor in Animal Behaviour.
Anything my Tony Attwood - he is a researcher into Asperger's and autism, and will give you hope.
Keep posting here and let us know how you get on. Don't be hard on yourself, just learn and move on. Guilt gets in the way and slows you down. You haven't got the time to be slowed down!
Marg