His feelings didn't flip so fast. He doesn't "love you".
Listen, I used to think this way, and it has taken some hellishly bad treatment from men to get me to change my mind, I wish I had listened to more experienced women earlier instead of thinking that I knew better. I have 20 years on you so take it from me.
When someone loves you, you see it in their actions. Words are cheap. They don't cost anything, and they certainly work very well at getting a girl into bed. Actions generally cost effort, and if a guy is giving words but no actions then you know which side your bread is buttered.
The guy is telling you about threesomes (to be honest he fucking wishes), he's doing this to make you insecure and get you to react emotionally.
You have fallen straight into this trap by... reacting emotionally. He now has a young, pretty, 20 year old girl chasing him around begging him to please let her go on holiday and saying "don't you love me??". How do you think he feels about that? He feels like Brad Pitt fighting women off with sticks. He's all like "nah i'm gonna go with my friends, and have a threesome".
Imagine for a second that you heard him say this, genuinely believed that you were exceptionally worthy and that you could have any other guy you wanted, and thought "yuck, that's gross, he's not really who I thought he was" and you just stopped texting him back. Whenever he asked after you, you were busy, far too busy for him. You were out having fun with you friends, you were planning your own holiday, or you were busy studying or doing things that benefited you.
How do you think that would go down with him? He'd think "shit, she doesn't care - nothing I am doing to upset her is working!!!".
This is how men work. This is the kind of behaviour a lot of them engage in because women are emotional, we talk to each other, we will sit for hours over a glass of wine chit chatting about our feelings and how sad we are or what colour lipstick we should buy. Men don't do any of that. But they do know that if they can TELL us things that make us feel amazing, we're likely to want to be around them and we'll sleep with them.
I used to think like you "how could he do that??" "doesn't he care how I feel??" - change your thinking my dear. The films are inaccurate, disney has lied to us, and men are not the same as we are. Treat them accordingly, flirt back, reward the positive things they do but for the love of God, ignore them when they start to try and create insecurities. You dont' need to say anything, you observe, and you walk away like a queen.
I wish i had adopted these principles far sooner in my life, because it comes across as someone who doesn't involve themselves in drama and is FAR too self assured to get involved in the games of petty men. It's a hard thing to do because we do get emotionally attached, that is how we are wired, but we have to resist that sometimes with men who are not making the effort for us.
How he treats you is how he feels about you, and the opposite of love isn't hate, it is indifference. Learn to be indifferent.