This is going to be a really hard talk for you both. But you need to say that you are aromantic, and you don't want to lose him by breaking up with him. I suggest the Oreo method. Say something very loving (about how you love him as a friend, because that seems to be sincere on your end), say the hard stuff, and then end it by saying something sweet to him about how wonderful a friend he is to you and how much you care. Say your relationship is going to have to change, but how you feel about him hasn't.
I don't know if any of this is good advice, but it might be? I'm just going by my own common sense here. Good luck.