I started my PhD in science about 11 months ago mainly because I didn't have a specific plan for what I wanted to do with my life after my Masters. A month into it, however, I got a call from one of the companies I had wanted to work with (I lost hope after not hearing from them for 3 months) and I embarked on a lengthy recruitment process which is still ongoing.
My situation is complicated by the fact that my supervisor is a really nice guy. He took me in despite crap grades in my first year of undergrad and lack of publications from my masters, and is now paying me from his own research funds (in Canada, it is extremely difficult to receive scolarships/funds without having published). On the other hand, the project I was given was very ill thought of and quite vague. I've tried to find a tangent to work on with limited success and absolutely no encouragement/support from my supervisor. He isn't pleased with the fact that I've diverged from the topic he wanted me to work on, but I have absolutely no motivation to go back to it, especially that I very strongly feel it's somewhat of a dead end.
I have started hating the lab and the very obvious lack of support (for instance, my supervisor will ask the other PhD student to attend conferences and prepare posters/presentations, but not once has he asked me) and I have to drag myself to work every day.

Right now I am fairly certain that I will be getting a job offer from the company some time soon. The pay is excellent, advancement prospects are even better. I will know their answer in September for a start date in November.
My guilt is eating me up though. I guilt-trip myself over quitting my PhD and how my supervisor would take it. I feel like I wasted almost a year of funds that could have gone to a more worthy student, but I just couldn't quit without at least having a solid prospect to move onto.

When should I tell him? How should I tell him?

Tell him/her/it straight up truth with tact: tell them you're not engaging with the work, you feel this career path is not for you and, while you sincerely appreciate all the time and effort they have put in, you feel it would be unfair both on them and you to expend further valuable resources for suboptimum gain in a field that leaves you feeling trapped.

Voila, damn I'm good 8-)

Your supervisor might be nice - but honestly, if he's not giving you opportunities, he really can't be that good. That said, yes, just tell him once you have a job offer. You'll have a difficult conversation, but then you'll be free! Imagine how good you'll feel! I have a restless employment history, and always hate telling the boss that I'm leaving, but they get over it soon enough. Your supervisor might then find someone who wants to do his original project. You have to do what's best for you and you can't go on doing something you hate.

And you have to publish from a Masters? Wow, we don't need to do that in Australia.

Must I do it in person? I'm so not the confrontational type and I already feel guilty enough... :-(

Yes, you must do this in person. He's been good enough to be your supervisor, and you should do him the courtesy of telling him in person. Don't expect it to be confrontational. It won't be as bad as you think - someone else on this forum said that the reality of going through with something they didn't want to do has never been as bad as the actual reality, and I agree with that.

Always remember that your PhD is just a stepping stone to your career goals. Considering you are going through this now i.e in your first year I would suggest tackling the situation immediately. You must tell your advisor in person, at the earliest possible time,  that either you are considering leaving or leaving for good. Advisors are understanding and this situation arises more often than you actually think. Forget about guilt. If you're unsure about your PhD plans this early in the game, the rest of the upward hike is going to be confusing, depressing and extremely frustrating for you. Good luck.

I hope it goes well... i know that if i told my supervisor i wanted to quit then she would convince me to do otherwise as they can be quite convincing. You should write down what you want to say and your reasons and then back this up with an email afterwards.