My male kitten has a little bubble under his skin on his belly?

Question
I have two kittens, a little boy and a little girl.  They were born on July 12, 2012.  I got both when they were only 5 weeks old otherwise the owner was going to kill them.  When he was about 7-8 weeks old I started to notice a little "bubble" under the skin on his tummy. I would say it is about the size of a dime right now, it seems to just stay the same size.  It is located just a bit on the right hand side when looking down at him while he is on his back, at around the place where his belly button should/would be.  When just looking at him you can't see it.  There is no visible signs that it is there.  When you rub his tummy though (which he likes) you can feel this little bubble.  It isn't hard, it doesn't seem to cause him any pain at all, he eats and drinks all the time, he is very sweet and loves to play fight with his sister, basically there seems to be absolutely nothing wrong with him at all.  I say "bubble" because it feels like there is just a little pocket of air right under his skin.  If you press on the bubble (even really lightly) it seems to go away until he moves and then it pops back out.  Any idea what this "bubble" can be?

Answer
Most likely, this is an umbilical hernia.  When the umbilical area does not completely close, there is a  hole that remains.  Fat can wiggle through the hole and cause the bubble.  When you push on it, the fat wiggles back into the abdomen.  A simple procedure to close the hernia may be needed.  Many of these will close on their own with age and development of the muscles around the umbilical area.