6yo with AuDHD and I need help!

Hi,
i have a 6yo son who has been diagnosed with ASD (Level 2) and ADHD a year ago. On top of that he has delayed speech and also just doesn't understand lots of things. I don't know if thats caused by his ASD but he has a very limited capacity to understand abstract things. For example he can't understand things like "yesterday" or "tomorrow", he doesn't know what it means if we ask "how are you" or talk about things that are not right in front of us. Is there a word for this?
He is very aggressive and violent.That started about 2 years ago. Everything that happens that he is unhappy about/doesn't agree with, may it seem minor to us, leads to major outbursts. He has destroyed so many windows, objects and made holes in walls, not to speak of the pain he causes us daily with kicking, hitting, pinching.
He has two sisters, one older and one younger.....

It all comes in waves.. we will have one good week and then one week out of hell basically.
Can anyone that has been through similar things please let me know what would help him and our family? What type of therapy would be suitable? He is not mentally capable of talking about emotions at all, I can not reason with him or explain things to him.
He is not on medication. This (Ritalin) was offered by the doctor who diagnosed him but we did not see how it would help and also we probably couldn't get him to take them anyway.
He does not go to school as we are a homeschooling family.

I hope it was all understandable, english is not my native language, please ask if something is unclear.