How do I get my autistic son to stop hitting me I'm his mother. Therapy isn't working.?

Usually it’s because he relied on you and feel like you are the only person he can express his emotion to.

Do you talk very differently to him and to other people?

You need to build up the “individualism” between him and yourself. Like what you would said to a equal friend, you need to said it to him. For example, if he said something/ask you about something he need, you need to reply him with some “individual tones” (if that make sense). That is without emotion involved in the sentence. In that way he would feel calm and do not feel like expressing his frustration to you.

Because as parents, you need to bring calm to his chaos, do not join his chaos or do not make him feel unheard.

Set boundaries. Like when he’s in a room, do not open before he answered. This is what’s needed to build the “individualism” too. When he started to see you as an individual, but not a person he’s entangled with, he will soon be calmer.

Please do not seek advice here for this! Look, years ago I was a special education teacher for autistic children in elementary school. You know far better than any of us where on the spectrum he is, and I’m sure doctors and therapists have explained things to you. Violence is common but it’s generally born out of frustration due to his inability to communicate effectively. I had students that could go from absolute calm to wild animal mode in a blink of an eye, and I’m sure you’re at your wits end. Don’t despair. Seek more professional help. I’ll never suggest institutionalizing him, but I know that sometimes there are adolescents that do better in such situations. Talk to your support group and avoid places like this for advice. It’ll only confuse you. I’m on this site because I like to offer help with homework and things related to universities, but I saw your question and remembered my life years ago before I went back to school to become a professor. Good luck! 

i'm sorry for your loss. i am currently in the same position as your son: i am around the same age as him, we are both autistic, both heavier than parents, both thinking they are not loved by parents.
the best thing you can do is trying to give your son a bit less attention than normal, and to see if he changes for the better.

Are you effectively disciplining him? He will need a strict set of rules (preferable written with some images so he can see them to help process it) and punishments such as time out, or prohibited from using a favourite item. There shouldn't be exceptions as it may make him confused, and it should be consistent

You must not puinsh him. You must reward him if he is nice.

2 tacks you might consider
treatment,,it is  not understood by the medical profession,the cause,,unknown,,treatments?,immune modulation,of course this road is untraveled by most in medicine,,those that do,,are expensive upfront
 11-1500 bucks due before the boys gets seen, ( american academy of environmental medicine)
the boys violent episodes may be triggered or worsened after eating,drinking trigger foods,,whether protein,carb, drink,,,food dyes

 colors,preservatives,,,trial and error,,direct challenge may help, as in taking careful notes,noting amount he eats ( small doses,very small does),drinks,the reaction,skin color change,pupil dilation,change in breathing, flushing on skin of the back,
mouth noises,,rapid speech,tremors,,,
you might discover one food or food group to be the offender,
mold may be an element in his reactions to you,
a poster put up a short vid on twitter a couple years ago,showing a 2 year old on a highchair,before and after,getting a hit off a joint,,mom was desperate,tried everything else,,the pot worked,,the kid was sullen withdrawn unresponsive before,,fits if anger,,and turns into an amiable chatty kid,,
in years ahead, the boy may have to be institutionalized

What are you willing to do to guarantee your safety? Hit your son back or have your husband hit him when he hits you.  Put a shock collar on him.  Lock him in a room.  Rehome him.

Punish him by taking what he loves most away temporarily. If he throws a fit, ignore it and do not give it attention but be mindful to make sure he doesn't cause self-harm. Show him love and compassion when he acts correct but when he doesn't you must punish him not with physical harm. There are two paths the 1st is to show dominance you are the parent he is the child 2nd is to learn to deal with him and make it a simple trade. As long as he acts right good things come to him when he acts wrong punishment is the result. I was taught growing up that if I acted a fool I will get smacked like a fool. If I acted correct good things come. I hope this helps good luck.