Hi S.~
I nursed my son until he was 7 months old, and really only stopped at that point because I had resumed working full time, and my milk supply just diminished to the point that he was more satisfied with the bottle!
As for your situation, I would suggest (and I am going to even surprise myself with this recommendation) contacting the LaLeche League for some advice. They are the guru's of nursing, so if anyone can help, perhaps they can. Another place I always look when I need info, is www. askdrsears.com. I don't know whether they will have anything specific to your situation on their site, but if you have the time, its worth a look.
Personally, I would probably say that it would be the smoothest transition if you cut back one feeding at a time, or like you said, eliminate the naptime nursing, and then, ultimately the bedtime nursing. Does your son use anything else for sooting himslef? Naturally, I would supply him with whatever that is to ease the transition. My son started taking a pacifier at 6 months, which at that time, was a lifesaver, because he constantly wanted to nurse just to soothe himself. Of course, now it's a nightmare, because we have to get rid of it, like yesterday... but that's another post all in itself. The point is, that it really helped us through that transition, but I wouldn't suggest giving him one now, if he doesn't already take one, but that goes without saying.
Also, does he accept a sippy or a bottle for milk? If so, he might not have too much trouble with the transition.
Best of luck to you... I know it's an emotional thing for both baby... and mommy...