If you are following the original bass lines, you are not overplaying, and I think you are entitled to say those words to him.
You can be flexible but firm in this case. The firmness is that playing original bass lines is NOT overplaying. You can be flexible in approaching future music, but you can make your own decisions about what gets played. You may find that sometimes he's right, and sometimes he's not.
If your guitarist won't budge in terms of accepting the original music, then I suggest you bow out before you get frustrated.
This... right here..
