Been there too. I have no real advice, because my experience in trying to hassle motivate my son was much less successful than Medusa's: it resulted in or coincided with (I don't know what was chicken and what was egg) my son rebelling big-time: swearing, shouting, running away, being very verbally and occasionally physically violent, taking drugs, etc... :( But if you can stand over him and it works, I guess it's worth a try...
However, I can perhaps offer some reassurance... My son did no work for his GCSEs, but because he is bright, he still did OK. Not well; not what he was capable of; but enough to have his choice of courses at FE college: 5 including English and maths. This is all they actually need to progress, and once they move to the next level, their GCSEs results don't matter any more.
When I say my son did 'no work', I mean none... Well, 40 minutes of revision at home in total (when I went through most of a past maths paper with him, before he climbed out of the window :( ) His attendance dropped to about 65% and he spent about 1 day per week in internal exclusion... (For some disruption, but also because the school's punishment for truanting was to exclude them
) They predicted him 2 Es but he got a B and 4 Cs. So bright kids can pull it off, even without work.
The other bit of reassurance I can offer is that your son still does have time to pull his socks up, if he wants to. In the November before my son's GCSEs, his head of year said he still had lots of time; in February, my sister in law, who is a deputy head in a high school, said he still had time... It's only September... You don't need to panic yet :)