This would really depend on your relationship with him and his mother, how long you have been in his life, what the rules are between his mom and you on how to deal with him, and many other things. Also one thing to take into account is how old he was when you came into his life and what you have done to earn his respect. While a child should give basic respect to everyone you don't just get to pop into a childs life and demand respect after they have spent time either with no father figure or someone else as their father figure. I have 2 boys from my first marriage that my husband has taken on as his own. For the first while he couldn't figure out why they had a smart mouth with him no matter how we dealt with it. I finally told him if he wanted to be treated like a father he needed to act like one. It's really easy to be disrespectful to someone who takes your mothers attention from you and doesn't give you any attention to make up for it. Do you attempt to do things with him? Play ball, take him to sports things, comic book stores, fishing, or whatever he is into? Do you know the names of his best friends? What his favorite movie is? Favorite song? Favorite place to go eat?
While I know it sounds crazy you have to make up for all the years you weren't in this child's life. You have to earn love to get full respect after taking a child's mother. You don't get to just be the guy who tells him what to do and nothing else. When you give respect to a child and their wants and needs then usually they will return the respect.
I was guessing based on your profile pic you are a step dad. If by chance you are a step mom, the same rules apply.
No matter what you are communication is the key. Family communication and communication with mom and son seperate. If you can't sit down and talk as a family something is seriously wrong.