^^ not saying this is a long term solution but if he is violent during check out because of having to let go of his item for scanning I would maybe bring a separate item of the same thing and hand it to cashier and say this one is for scanning my son has one but this one goes back?
Sometimes just tear off the UPC or tag and hand it to the cashier

when my son was younger he had issues with the check out process. what we did is we took him to a smaller store for a small fun store run. (like ice cream or other treat) then i asked the cashier if he could stand by her to see what she is doing. he saw how she pushed the button to make the belt go and how she scanned the things in. it has made it easier after that. the not knowing who has control of the belt was the trigger for him. he thought the belt just moved by itself and thought it would get him, but once he saw the cashier controled the belt he was fine.

My grandson did the same thing, at first going to the store he would want several things and would get unbearable if told no, when I would take him I would tell him he could only take home one thing,no matter how many he had in cart he had to choose which one he really wanted and that finally seemed to work, until I went to the store with him and his mother and he wanted several things and his mom was saying no and he had a fit, it upset her so much she went out of the store in tears when we got home I tryed my best to explain to him that you always cannot have all you want and also that unless he behaved in store well there would no anything and if he ever acted up like he did with his mom well then he could not go to the store, well long story short,he no longer tantrums in store wen the item is put on the belt(still do not know why he did this)Nor was he tired and ready to leave cause he got someting)only because we have tryed and tryed explaining to him and this has taken a while.That is the way we got thru this phase as I call it and I am quite sure all of these children must experience something but what it is dont know but things have changed and I always remind im before going the rules.

Make a visual flip book of exactly what will take place. You would need to take pictures of each step of the shopping process. He can flip or mark off the pictures as each step is completed. When my son was younger(he's 12 now). Even though I would verbally tell him things, having visuals of what will be occuring helped.

Ask HIM what's wrong. Ask him to draw what upsets him. He just might tell you himself. ;) mine did.