What I know is that several self-storage places around central CT do rent bays large enough to store cars in. They are neither heated nor alarmed, but they are fairly secure. Some don't allow cars to be stored in them (gasoline and oil and fire concerns) but customers sneak them in anyway. Others require you to reasonably drain all fluids before storing on their property. A 10ft x 20ft bay is the size of a one-car garage and many of them rent for little more than $100/mo.
If your boss has any kind of a classic Mustang worth displaying, he could get it detailed and ask to park it in the lobby of his local Ford dealer. I've seen many nice classic cars displayed in their marque's showrooms over the winter, including an Anniversary 280ZX which parked at Executive Nissan one year. Customers love to see the old cars of the brand they are buying, and dealers love to point to them to show off the enduring quality of the brand they are selling.
If he gets really desperate, he could just park it under the garage at the airport parking lot and return next spring
. He can tell the gatekeeper he lost the ticket ~ they can't tell how long the car has been there so they'll charge him the "MAX" for a single day, which is hugely cheaper than $100/month. Do NOT tell him I told you to do that.