R.H.
I was all set to say, "oh, he'll outgrow it. My 3 year old till sucks his thumb" then I read that your son is eight. But even so, I don't think that would bother me too much.
Is it affecting his teeth? Because, if not, I don't really see a problem with it. He obviously knows it isn't really the done thing, to suck your thumb in public but if he does it in the privacy of his own home, does it matter? I know several adult thumbsuckers. You would never know unless they told you. And thumbsucking is probably preferable to some other habits anyway. That said, most children grow out of it at some point and I am sure he won't be sucking his thumb during job interviews.
Also, I think drawing attention to it won't help. My parents nagged and nagged me about nail-biting and thumb-sucking and all it did was make me feel annoyed and I ended up doing it more. If you can ignore it, perhaps he;ll stop when he is ready?
Sorry I don't have a magic solution. Good luck.