Certainly you could turn wireless off, and add a cable (if necessary, move the router to the lounge so he can use his laptop via the cable).
If he's not using the cable, but still "busy" on laptop, then he could legitimately be writing / studying things already downloaded, or he could have hacked into a neighbour's wireless router (even if he's not that techie, no doubt a friend will show him how).
Think you are fine about confiscation for a day or two, to show him you're serious that simply lolling about all day is not going to help him much. As someone else suggests, though, "Maybe a chat with him about what he wants to do as opposed to what he feels is expected of him could be a good idea?"
If Uni isn't his real goal, then see what is in his "desired" future.
FWIW, I never took 'A' levels.
I left school at 16 as I was planning to be a Merchant Navy radio officer, earning a small fortune and getting umpteen months leave each year (*)... That was the plan, then in the mid-70s the supertankers came on the scene, and oil firms had far too many staff, as they scrapped the small tankers, so the chance of 6 months probation on oil tankers or cruise liners became almost impossible with an excess of qualified crew... Luckily while doing radio course, had also done some computer programming and in computing, I got a degree equivalent, thanks to working in education and having my study paid for...
30 years on, not touched radar or ships' radio at all, and do I regret missing 'A' levels or university... a bit, but not much as I've been pretty happy with IT for all that time!
(*) my old head was a statistics gatherer, so I wanted to send him postcards from around the world to "get up his nose" about the "one that left with just O levels" !