If he is the one paying you, then he should be the one doing the billing.
If he is doing this then he can add his percentage onto your bill.
This way he gets his cut.
If you are billing the customer direct then he is taking the 'p*ss' if expects a regular cut off the top.
So if he wants a cut, bill him what you think the job/s are worth, make sure he pays you straight away and let him have th hassle of collecting the money from the householder, how much he majes then depends on how much he wants to add onto the top of your price.
No way on earth does he deserve a cut of any regular window cleaning work that you are billing customers direct for.
Talk about money for nothing.
Ian