I'm thinking of staying with him even if he steals from me.
What makes you sure that he is stealing from you?
He's a young and he makes a lot of stupid mistakes. He just loses my stuff sometimes and claims to not notice it.
Making excuses for your boyfriend's theft is just rationalization and you can't trust him because he's a thief. Get away from him and find an honest man to make a life with.
He cries when I try to break up with him.
Why would you trust him anyways? If he steals from you, you should just break up with him. There is no need to stay with a person that steals from you.