I think he had feelings for you, or he hoped they might develop. Might have been too early for that but that's my guess. Finding out you wanted to exile him to the Friend Zone was a blow to him.
Unless you want to build your house in the Friend Zone, you need to tell him you want to be exclusive. If he says 'no' isn't that something you need to hear?
My mantra. When someone tells me "I can't choose between you and X." My reply is always "I'll make it easier for you; I'm out." I often say, "I'm not interested in winning 51 to 49."