L.A.
Do you mean socially immature? Intellectually immature? Interests are immature? Is he a goof ball? Is he kind of Geeky and awkward? Is he a bit anti social because he is shy? ?
Does he have any emotional differences? Intellectual differences?
We need more information.
Boys are way more immature than girls at this age. If he has not spent a lot of time around older kids, he may not realize how he is supposed to behave or act. His hormones are going crazy, he can wake up one morning and not even recognize his own body.
Is his dad around? If not is there some man in his life like an Uncle or grandfather that can speak with him?
If he feels awkward, that does not help, he may want to find a sport like Tennis, fencing, swimming, something to give him a focus and some team mates.
If he is into gaming, have him attend some games at the local comics shop.
Start giving him more responsibility. Mowing the lawn, coming up with a menu and preparing meals, Cleaning the bathroom. Acknowledge when he makes good choices or does a good job.
If he is good with technology, come up with a project the 2 of you can work on and let him teach you a skill. I always asked our daughter how to do things on the computer, heck when she was 5 she taught me how to get on line and how to send an email! I still ask her for help with my cell phone, seting up all sorts of things on the computer.
Heck i know at our local library, teens volunteer with Senior citizens on how to go online with the computers.
Some people do not understand or even realize social cues. He may be a person that you need to actually sit him down and explain them. Especially only children that are boys. They just do not have any experience with what other kids do for fun, how to talk with each other.
Now if he is emotionally immature, like throwing fits, crying, pouting, etc. You again need to sit him down and explain proper responses to him feeling frustrated, embarrassed, angry, upset.. and then help him come up with solutions.
I recommend this book all of the time because it has basic information in it about being a teen and what is socially acceptable behaviors in our society, but is funny also.
How Rude!: The Teenagers' Guide to Good Manners, Proper Behavior, and Not Grossing People Out Paperback
by Alex J. Packer