Hey C.,
In your post, you don't say whether or not your pediatrician concerned about your son's continued finger sucking. If your pediatrician isn't concerned, maybe you should relax about it for now. You wrote that you don't think it's just for comfort, but that may be a part of it, even if you think it's mostly habitual. I have a 4 year old binky girl. We took away the binky when she turned 4 in April , after saying we were going to do it when she was 3, ha ha. Anyway, she did fine for a few weeks, and then she started stealing her brother's binkies. We're letting it go for now, telling her we'd really prefer if she'd let 'em go. Anyway, she's got some dental work in her future, I'm sure, but we're just trying to talk to her about it and impose some limits as far as the time goes-she can have a binky at bed, but not during the day, etc...When your boy goes to school, he may stop on his own in order to fit in with the other kids. I know my girl wouldn't be seen with a binky at her preschool, but for now, if it isn't causing any serious physical problems, and he seems perfectly well-adjusted in every other way, maybe you should stop worrying about it. Did the ped say anything about his fingers? Also, you didn't say if you've talked to him about why he sucks his fingers. Maybe you have, but if you haven't, that might be a good idea.
Anyway, that's my two cents. Good luck, however you decide to proceed. As he gets older, he'll give it up on his own, or at least only resort to it during stressful times. And who among us doesn't need some sort of release during stressful moments? Mine's chocolate:)!
Take care,
A.