Hi smiley, I could have written your post a year or two ago - except all my DS1 was ever offered was a bit of cash-in-hand gardening or odd job work, never a 'job'. :(
Ime, there's a big change when they hit 18. At 17, many of them are short if money, and they can't go to pubs and clubs anyway, so there are always a few mates to hang around with on a Friday night, who are equally broke and happy to club together for a few beers or a bit of weed to share. At 18, suddenly there is much more they can do socially, and many more if them get p/t jobs, and the ones that don't are left out socially, and start to feel the strain. For my DS, his 'wake-up call' was Leeds Fest: almost everyone he knew went, and he couldn't, because he had no money. He had a miserable lonely weekend, and vowed then to get a job so he'd be able to go next year!
The other thing to consider is whether you give him money, and how much. Over the past couple of years, because my son was at risk of dropping out of college, I gave him £20/week if he attended and did other jobs around the house. It worked to keep him in college and occupied, and I don't regret doing it, but it meant he had cash each week. After a couple of years doing this, I told him when term ended last year that I would no longer be giving him money in September, and I stuck to that, which certainly helped motivate him to finally get a job.
When he started a job recently, and announced he liked earning so much he didn't know why he hadn't done it before 
I couldn't resist pointing out that is been pushing him to get one for over a year. I was a bit shocked by his response, which was that I'd been telling him to get a job, but I'd kept on giving him money, so he hadn't actually needed to do it! 
One last word of warning: don't blame the girlfriend, or make any other excuses for him. At 17, he has to be responsible for his own actions and mistakes: if she distracts him, that's because he lets her! Life will always be full of distractions from work; we all have to learn to ignore them, or manage them, and go to work anyway! Your DS will always have girlfriends, or friends to hang out with, or other distractions and things he'd like to do... He needs to learn that he must still go to work!