Set up a style.css file somewhere else that he does have access to.
Then change your style.css file to read:
Your style sheet will then load his style sheet.
I'm not sure that this will work from one domain to another. I don't have the time to test it.
When he is finished making modifications, be sure to replace your stylesheet with his.
Thanks for the suggestion but I am a bit skeptical about this working. But this is worth a try. I just do not wish to mess up with my live site at all.
1. Ask your host
2. Use http://pastebin.com/
3. Create a test site similar to yours and give access.
Cannot understand the point #2 I have seen that site but not sure how to use it. But the 3rd point looks better. I am thinking of creating a new site (test site) and load it with all the files that I have with my other live site. Then give full access to that to my designer. Hope this works fine. Once all done, I may copy and paste that info onto my live site. Makes sense?
Thank you very much for your suggestion.