If there's a written dress code saying something like if you wear sports-team attire, it must be for a Chicago-based team because of business reasons, and he violated it, then yes, he could be terminated for not following company policy. There also needs to be other factors congruent to specifically selling cars/dealer services.
If anyone's shallow enough to avoid a dealership that a salesman has other than a Bears tie on, then they probably aren't worth talking to in the first place. That car dealership owner is a moron.
The salesman could own a few of that dealership's cars after all this if there was no bonafide occupational requirement that he not wear anything other than Bears gear. I don't think Illinois is a right-to-work state. Plus, if he was under a contract, it would depend on the conditions. It'd be interesting to see how this pans out if he litigates.
Of course, if look at the state, you can see a glaring issue right there...Illinois. The land who boasts that they had the "Great Emancipator", yet ironically, wants to control every aspect of your life with government intervention/nanny state mentality. I refuse to claim I was ever born in that sorry ass state. It's embarrassing.
2010 2SS/RS Auto, Sunroof, Engine Cover. Transformer Edition package.
2013 ZL1 ragtop #5478. 1 of 54 CRT M6 'verts.