Relationship rant.. Don't know what to do.. Help 😩

So basically I'm seventeen and in a relationship, it's our one year anniversary in May and to say we've hit a bump is a bit of an understatement.. 

At the beginning when I got with my bf there were some drug related issues which resulted in him being arrested. He is in foster care so was putting his placement at risk and as a result our relationship as well... Well he made me all the promises about ill stop you mean more to me than drugs, I don't want to lose my placement I love you blah blah you're the only thing I need and six months down the line we are back in the same situation. 

I went round today and he told me that he'd gone out last week and was smoking weed and in addition had tried a format of ecstasy called MDMA .. He flat out lied to me over and over to my face and only told me because his cater had threatened to tell me if he hadn't.. I love him, but I can't stand the lies and the deception. He is going to be kicked out of his placement with his carers over it as it turns out on top of the weed and ecstasy he had been caught smoking weed in his bedroom two doors away from both his carers and their new foster child.. After the promises he made me in the first place, and all the lies I don't feel like I am trust him. He had begged me to stay and all that shite but I just don't know what to do.. My heart says stay but my brain says go because he keeps on making me look like a mug.. I'm not sure I've explained this very well but just wonders what you guys would do in my situation or if you'd ever dealt with something similar before.. It's just I'm quite young and it's the first time I've ever had to deal with something like this.. Thank you in advance you beautiful people xxx