Wait a minute - that is not fair! He needs to be able to keep 100% of his earnings, plain and simple. Every human on this planet should be able to say, "I am available for work today, this is what I can do and these are my terms and there is my hard work involved -- if you like, then ask for my service, and I will pay the platform how I see fit". For the simple act of connecting me with a customer who is willing to pay me for my hard work, a 20% commission paid to an app is not ok with me and there need to be better, more worker-focused alternatives available which still connect me with a global marketplace but let me dictate the terms for my hard work.
This is the manifesto for this enterprise - empowering humans to be their own absolute boss and re-setting the terms in their favor. The app is the servant, and the worker is the master.
An initial version of this vision, "Runnr", is available on both iOS and Android, built with a lot of love. It does almost everything which a typical on-demand app can do and more. The focus is very narrow right now - on students only for jobs in their own neighborhood in a limited geographical area. Check out http://www.runnr.ca