I'm at the end of my rope!

Long story short, my boyfriend has a 3 year old boy who's life I've been in since he was born. We have 50-50 custody of him (2 weeks on, 2 weeks off). He is usually the sweetest little guy, always well behaved, only throws tantrums every once in a while, etc.

Well we just got him back from his mom this week and he has been an absolute terror. He literally screamed at the top of his lungs for a good 30 minutes last night because I tried to give him a green sucker instead of a red one (he has never cared what color his candy is before). He finally went to bed, and this morning the first thing he says is "I want a sucker."

Well, with it being 7am I told him if he was good at the sitter today he could have a sucker when we got home. He started freaking out again and then told me "I want you to go away forever!" I have never heard him talk like that and it broke my heart because I am trying so hard to do everything i can to make him happy, I have since he was a baby.

I asked why he said that and his response was "because you won't give me a sucker!" We then called his dad on speaker to have a talk about being nice, several times he said "I dont care, dad!" Its just freaking me out that he is only THREE and he is acting like this, all of a sudden too.

Its making me question why I'm staying in my relationship also, I dont have any kids of my own. I love him like he is my own child though because I've been around for his whole life. I would do anything to make him happy and keep him safe. I just dont know what to do anymore.

Has anyone else here dealt with this kind of sudden personality change in a 3 year old? Did it last? Help!!!