Why is he like this?

I like guitars lol well idk maybe he is just a flirty person or he just wants to hurt you by doing that or maybe he just wants to be friends and since he is your ex whatever he does seem like flirting but maybe he is just trying to be nice

Don't talk to him anymore, if he calls tell him you don't want to talk. he is doing that to try and get under skin a little, try to ignore him to the best of your ability. if he still likes you he will come around then make your decision and let him know how he has made you feel then go from there.

Because you let him continue to hurt you. Tell him that you can't be with him if he treats you badly, and doesn't feel the same about you, as you do about him. His flirting is his way of manipulating your feelings, not showing you he likes you.

He's using you don't let him and say cut the flipping string

Because you allow him to. Sorry sweet but that's the truth. If he says he doesn't have feelings for you then why is he flirting with you? And more to the point why do you succumb to it? Love Hurts> Clearly it hurts you. Seems to me he is playing you. Move on.