ok i have a few points of note on this which will be quite helpful.
1. How has he connected the computer? does it only show up on 'My Network Places'? or has he connected a drive?
2. he cant connect directly to your laptop without setting up wireless up as ad-hoc(if this was the case your computer shouldn't connect to the router.
3. he couldn't of got on your wireless network without your wireless key, simply CHANGE IT.
4. in the majority of countries using someone else wireless connection to access the internet is a criminal offence, you could threaten/take him to court.
to me it just sounds like he connected to your laptop, but isnt actually using the internet (or is actually still connected to it) but you havnt explained your problem very well.