Manny:
You are asking how to send source code from one freelancer to another.
This simply isn't an Upwork question.
Upwork actually does not place any restrictions on how you do this. But also, Upwork isn't a source code sharing utility or code repository system. Nor is Upwork a file sharing system.
re: "I am worried that he might just keep the code for himself. Is there any other way to safely send the code files?"
Of course any freelancer you share source code with could "keep the code for himself."
But this is usually nothing to worry about at all. Because freelancers you hire on Upwork aren't interested in obtaining your source code. They are here because htey want to be paid for work that they do on your behalf.
If you accidentally sent me your entire body of source code, then I would do the same thing as nearly every other developer on Upwork: Ignore it or send you a note pointing out that you accidentally sent me those files. I wouldn't be interested in "keeping" or even looking at your source code. It is of no use to me.
If you want to develop effective strategies for sharing source code or for archiving or managing source code, this is something that you can do research on for yourself, or you can hire freelancers on Upwork who can help you.
