I would recommend the following:
Robert Muchamore - both the CHERUB and HENDERSON BOY'S series. I read the Cherub books myself and they are great little reads. Both are very action based and aimed at teen boys.
Jim Butcher - Dresden Series. A long series with quite short books about a wizard. Fantasy with some action. I would check one of these yourself to make sure they are appropriate. I wouldn't have any problems with a 12 year old reading but some more conservative people may do, there are some sex references but nothing graphic. Again, these are books I've read and loved.
Malorie Blackman - The Stuff of Nightmares - Adventure/Fantasy book about a teenage boy who encounters death. Not read it but on my to-do list.
Narinder Dhami - Bang Bang Your Dead - Adventure/Action based on a girl who's class is taken hostage by a gunman. Not read but on my to-do list.