How long has it been since his dad and mom divorced? How long have you been in the picture? The duration of your relationship with this boy, and the feelings he has about his dad leaving his home, would probably determine what you can reasonably expect from him.
And this is pretty common behavior in many adolescent kids, even when a step parent isn't involved. Since you have short periods of time to get to know him, take an interest in his life, and establish a meaningful relationship, you're at a disadvantage.
Your embarrassment and irritation with him could actually be the impetus for ongoing annoying behavior, if he resents you or is still angry or hurt at his dad over the divorce. He didn't ask for those changes in his life, and they are almost certainly painful for him. Your expectations that he be a man are probably pretty out of reach for him most of the time, especially if he experiences your attitudes toward him as judgemental or antagonistic.
I'd like to suggest a really great book that has tips step-parents would find effective, as well as parents. This child may need to know you're willing to be on his side and support him where he is, rather than demand he get on your program. Really, do you give him reasons to want to? But you can find out how to respectfully support his emotional state while asking the same from him by applying the wise techniques in How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. Every parent I know who has employed these tips has been amazed at the transformation in their children's attitudes, and their own.