You'll have better success advocating for open source to your manager if you focus on the concrete advantages most likely to interest them. Money talks - put together a simple spreadsheet to show the savings you'll achieve with the project you're recommending. Don't forget to include the client access licenses, since they can really add up and many people rooted in the Windows world sometimes don't realize how much they spend in this area.
Solid evidence of how you can save money with open source will speak louder than most evangelizing.
