Hi louculhanane, let's take a trip to OMG!! strange but true land. My daughter was plagued with warts on her fingers 3 - 4 years ago and we tried all available medical remedies.
Nothing worked until one day her gran said "oh we used to suck them and they'd disappear".
I know, I had the same reaction but guess what? A few weeks later no more warts! I appreciate that it's going to be difficult to get a whole hand in his mouth but maybe consider spitting on them and rubbing it in. What does he have to lose?
Hope it all works out for him, good luck.