Here's a solution, this is your scenario exactly. A different iTunes library for each of you, then your stuff won't mix in with each other.

If you want to use any music that's in your iTunes library you'll have to import it into his since you'll both have your own space.

http://www.imore.com/how-use-multiple-apple-ids-one-computer-and-itunes

When you want to switch libraries you just have to Quit iTunes and relaunch while holding Option to choose the Library.