Im going to dish out some tough love.
I cant imagine what it must be like to be widowed with a young child and be dealing with this by yourself. But because its hard isnt a reason to give up. You owe your son that. Dont pussy foot around him because some children act out when theyre grieving. Children arent stupid and they will simply see it as a green light to do as they please. He is testing you.
As a pp said there are 2 issues here. You need to be in charge, and you need to get to the bottom of where he has learned these phrases. It is a sign that he could be being abused. If he wont tell you, then get an appt with the safeguarding person at his school. If youre not happy with the outcome then i would contact ss directly. Someone needs to get it out of him.
If you are struggling to be the authority then you need to learn how to do it. So read some books (how to talk so children will listen is a good one) and see if you can find some parenting classes.
Id also keep a diary of what happened and when, who you contacted and how, what was said/agreed. It can be hard to remember everything all the time.
From your posts its tough to gauge if you understand the urgency. These things need to be dealt with now. They wont go away by themselves, its more likely they will get worse.
Have you got some support in RL?
Im sorry if I have got the wrong end of the stick and have been too harsh. Your posts are quite vague so it really is tricky to guage.