I think Chicory explained it pretty well. You DO want to get an experienced professional, and not try to do it yourself. Done incorrectly, an intervention can do more harm than good.
About all that an intervention will do, though, is get someone to agree to go to rehab. I don't know that it increases the chances they will actually stay sober. If he's already "done rehab" a few times, I'm not sure how much good another shot would be unless HE decides he's ready.