We had a similar experience with our three. Luckily for you chaps, I'm guessing your lad's level of understanding's becoming really strong now. We made our's a "deal" when it came round to their birthday.
They all had a present that they really wanted. We proposed that they swap the dummy for the present. His decision would be final and the dummies would be posted to Santa to let him know that little johnny was being very good and grown up.

Worked for us

Hi,

I don't see the harm him still having a dummy when he goes to sleep. Who sees it anyway? I sucked my fingers until I was 12, its a comfort thing and you can't take away thumbs or fingers. Maybe one day he'll just not need it anymore?

Hi Martine,
I sucked my fingers too. I was around 16 before I stopped. Unfortunatly I ended up with crooked fingers and front teeth. As a child I also picked loads of stomach bugs through putting dirty fingers in my mouth.
You're quite right though, it's whatever suits the child at the end of the day and it's hardly like he's gnawing the bed post. My parents tried all sorts of things to make me stop (from mittens in bed to sour tasting nail lacquer), nothing worked.
Our experience is that we don't often give our kid's self control the credit it deserves. It might also be worth playing on any childs aspiration to be more grown up. At the risk of sounding preachy, appealing to their sense of self respect (especially compared with siblings or peers) was really effective for us.