Too confusing: My friend's ex-bf, me, and my other friend...?

My friend broke up with her boyfriend because she thought he was too controlling. After they broke up, he refused to leave her alone so she asked me to distract him from her because she knew he'd started talking to me right after they broke up. So, just for the heck of it, I agreed.

Before long, however, it became more than just an entertaining project, me and my friend's ex actually became friends. Then, I developed feelings for him but I fought them because I would never do that to my friend. But, eventually, he asked me about it and I, being a generally honest person, told him that I liked him but didn't want to. His ex, meanwhile, said she didn't care what we did. He said he understood and was cool with it and that he actually liked me back.

But the problem was, he always talks crap about my other friend, K. Whenever I would try to make him knock it off, he'd get all mad. So I invited K to hang out with us a couple of times so she could stand up for herself. They already texted each other fairly often anyway so it made sense to me.

Now, however, he always makes excuses and doesn't hang out with me. We make plans to kick it and he never shows up or even tell me he's not going to come. Then, he'll say he's going to take a nap or he's losing service and stop texting me. The problem with that is K is my best friend and I'm with her most of the time so I'm perfectly aware that he continues to text her after he starts to "take a nap" or "loses service." Also, he's always telling me how busy he is and then making plans with her, which she then tells me about. K is a good friend, she always wants to make sure that, if she does hang out with him, I won't be jealous. And I'm not really, I know she's not going to go after him or anything, but it's frustrating. Plus, when he makes plans with her and then can't make it, he calls ahead and lets her know; with me, I never find out til he hasn't shown up.

I am so frustrated. I don't know what to do or say and I don't care if K hangs out with him because I trust her and she's a good friend. But I honestly have no idea what to do about him.

He's always the one who starts conversations with me but then makes excuses to stop talking to me.

What do I do?