Use the term zealot then, or something even milder (like "supporter"). The problem with a term like fanboy in a serious discussion is that it is heavily derogatory, and derogatory terms typically shut down debate.
Typically whenever people start throwing insults around, the debate has already shut down and people are just scoring points from the audience. The use of the zealot/fanboy insult is just to point that out to the audience in case they missed it, since someone can still be completely polite yet still a fanboy unwilling or unable to spout anything but their own set of talking points. So shutting down a discussion isn't necessarily due to the person who cast the first insult, and sometimes there wasn't one to start with.
It's the people who insist on being polite that you have to watch out for; you can be a lot meaner, with polite speech, than with derogatory speech.
It's the people who insist on being polite that you have to watch out for; you can be a lot meaner, with polite speech, than with derogatory speech.