Of course you could: and of course he could edit the source code and remove it as soon as it is on his server.
If you want to go down that route then keep all the source on your server and only have a script on his that calls it from yours, either as include files or such like, or as html content stored in the db, or even just as redirects to your server.
But even then, he will be able to just scrape the html from his browser once the site is up and running. We just did that for a site written and hosted by someone else because the partners in the hosting company have gone their seperate ways and the client did not want to be left stranded while they fight over the business. Now at least he has the html, images, etc if the worst comes to the worst.
It all depends on just how dynamic the html is.