Drug addict boyfriend

Please bare with me, and any advice or comments are appreciated. I've been with a drug addict for two and a half years. In the beginning i was very young, in the party phase. I was about sixteen years old, he was eighteen. We fell in love within the first two months. We spent a lot of time together, he bought me gifts. He got clean from k2 (synthetic marijuana) on his own half a year into our relationship. We did a lot together especially with his family. Since the end of last year he's been messing around with Meth and heroin. He's in straight denial. He claims he doesn't have a problem, he can control it, etc. I'm now eighteen and he is twenty one in a few days. He denies being high all the time, but is open about dealing what he obviously does. His dad enables him, because he is giving him a roof over his head, knowing what is going on. His dad always asks me what he should do, and i tell him to kick him out but i think he will always be too afraid to. He has no reason to quit. He has me, he has a roof over his head, and he has a way to make money for that next hit. He doesnt shoot up either. He snorts heroin. Claims he will never shoot up, because he's not stupid. He is a nice loving guy when he is sober, but when he's high he becomes this person that i can't stand to be around. We fight all the time. We never used to fight dirty. But it turned to name calling and it's just ugly things we say to each other. He's been dependent on drugs growing up. I'm pretty sure he's started doing drugs since he was thirteen. We're in a cycle. I keep leaving him but i also keep coming back. I can't stand to sit and watch him nod out all the time, and stay up all night. But yet i love him, and i don't want him to feel like I'm giving up on him. I know he's capable of better. In the first year of our relationship he got off spice, graduated and got a good job. He hated the job though because he was apparently harrassed. Eventually he got fired from being late to work all the time. Now all he sees himself as is a drug dealer. He thinks he doesn't have a problem because he doesn't have to steal. I've given him a million ultimatums. I don't know what to do anymore.