Hi
I have a bit of a different opinion on this.
Your stepson is your family and I believe that as parents we have a duty to help our youngsters get the best start in life possible. This also means making sacrifices.
He has a van which is a big help. During the next few months you can help him kit it out with the basic kit he needs to clean windows. (If he needs anything special - longer pole etc he can always borrow it from your van for the day).
Give him some of your work to get him started and help him with it until he gets the confidence to continue by himself. Help him with new customer quotes and show him how to go doorknocking to expand his round. Yes, you may need to spoon feed him for a while, but then you gradually let go until he can stand on his own. Yes, you may be short of work yourself to start off with, but it doesn't take long to replace your donated work. As I said, having children means making sacrifices.
Let him be responsible for his own round and give him 100% of his takings. Yes he needs to pay toward shared expenses, ie water purification etc., and being self employed would need to pay his own taxes when due.
We have a father and son working relationship and work this way. He helps me with some of my customers and I help him with some of his. If I do a day's work with him on his customers, all the proceeds go to him, but it works the other way as well. We also go collecting on a Friday evening together and collect each others debts. Our customers know both of us.
There will always be niggles in working relationships but it works for us.
Spruce.