I play airsoft regularly (once/twice a month) and it's a great hobby as long as you don't take it too serious.
I gives me a chance to run around pretending to be Jack Bauer, shout things like "Tango down" and just forget about the weekly 9-5 job.
Yeah sure it's geeky, but then i've been known to wear lycra.
Games are held on private land and are very well supervised, there will be an extensive safety briefing, and strict limits on the power of the guns (marshalls will remove and ban anyone caught with a gun over the legal limit).
However, be careful about buying him a gun. The law has changed in recent years (violent crime reduction act 2007) and buying a replica/airsoft gun is now illegal. The only way to legally buy an airsoft gun is to prove that you regularly play airsoft, this is usually done by attending several games and getting them signed off by the marshalls (google UKARA for a better explanation than i could give you).
This might help you find a local site
http://www.airsoftmap.net/UK/