
It depends what sort of thing your son is interested in. If he likes physical activities then an indoor climbing centre may be of interest to him -
http://www.revolutionskatepark.co.uk/climbingnews.htm it is at Broadstairs but not sure which part of Kent you are in.
or this link will give you a whole host of activities to choose from depending on what sort of thing your son likes
http://www.dayoutwiththekids.co.uk/search.php?county=kentWhatever you decide to do with him I am sure he will have a great day out :)
Snowboarding and snowtubing at Diggerland is quite popular with teenagers :)