You need to make some massive changes here otherwise he is going to get worse.
He needs to stop that pound for anything rubbish. Explain to him that you do certain things for him - such as feed him, maintain a roof over his head, clothe him, ensure his safety, etc etc etc (the longer the list the better!) and you do those things as his mother and as a family. He, also, has responsibilities as part of a family, and they are to make a reasonable contribution to the family, by way of a certain number of chores etc etc
It is only things above that, that you are offering to reward him for.
OTOH, if he would like a system where you each get paid for everything you do for the other, you suggest he gets on ebay and sells a kidney because he is going to owe you millions! I say that only because he needs to understand that you do so much and it is not a case of him doing everything! He needs to acknowledge all that you do. Not be grateful for it necessarily but stop seeing himself as some sort of drudge 
I would stop paying him to do homework. He either does it or not, but the consequences of not doing it will be borne by him in the form of detention, etc.
You need a total rethink on how you're dealing with him, tbh.