Good Morning,my son is also six and he has the same tantrums and meltdowns, and because he cannot communicate what is wrong, either myself or hubby just holds him firmly until he calms down. Sometimes that is the only thing that will work. Also, try offering his favorite thing, that sometimes works as well. But he can't have the thing until he has a calm body(I learned this from his ABA). I know exactly what you are going through. Best of luck to you and your son.

If the screaming is at night and he doesn't respond to you it could be night terrors. My son used to suffer from them. The doctor had me put him on a daily schedule. Also he was having a terror I was told to leave him alone. During a night terror he isn't really awake. So the harder I tried to calm him down the louder he would scream and cry. As long as there wasn't anything he could hurt himself with I just had yo let him cry/scream until he went back to sleep. Once he was on a daily schedule he stopped having them.
The tantrums during the day could be because he is frustrated with not being able to communicate. Do you use PECS and story boards? If not it may help.