As tough as this transition seems to my son, I think my wife is in worse shape. She was a teenager when her brother was born. Her whole outlook on male puberty has been observed from a distance. She's never seen the rapid growth, voice changes, and overall hormonal chaos in such an intimate manner. Frankly, she is terrified. Every once in a while she will cry for what appears to be no reason. When I ask her what is wrong she replies that he's "growing up to fast" or "acting too much like a man". I try to explain what's going on and prepare her for what's coming next which does little to ease her pain.
To all the moms out there, don't let this happen to you. Realize that they will make major changes during puberty but in the end they will still be your baby, every real man loves his mom. Let him grow.