Hi belfastmumxx, sorry to hear that you are having such trouble with your DS. I have a 9 yeard old DD myself and a 7 year old DS, and I must say, it is DS that gives me the most trouble, and it is ont unlike what you describe, just a bit more immature ;)
You must be doing a good job seeing that your son is well behaved in public! He is testing boundaries and "getting it out of his system" at home, which is actually a healthy sign, in a way, he feels safe enough to let his guard down at home.
But it's hard for you as you are not there when he is at his worst. And I think that is part of it, that you are not there. I think he is reacting to you not being there. I have done night shifts myself, and it's hard. Does he know you are plannin on going back to college? Could he be reacting to this?
Now I know that if I am right, then you will feel frustrated as there is not much you can do about it (I work full time myself, and it's obvious when I work too much, it's reflected in my childrens behaviour).
Your mum needs help with the boundaries. Get her to enforce no screens after dinner or 6pm, which ever comes first. That would be a great start. It will help your mum regain her authority and it will help your son sleep better, which will improve his behaviour.
When you don't work (wold that be weekends?), make sure that you do things with him where you have a lot of eye contact and communication, so he really fills up on your attention. Board games are great for this.
I also suggest you ask the school if they have a Family Support Advisor. I have used ours on a few occasions and she was great and really helpful.