Who does not know what the traditional iconographic image of St George look like? A horse, a spear, a dragon. And... a small fellow with a teapot (or something like that). At least at this particular painting in Sozopol, Bulgaria - and I think I've seen the teapot guy elsewhere as well. Any ideas? The teapots (or pitchers of similar kind, if you wish) are of some importance in some Islamic cultures: e.g., you can see them outside of many Muslim restaurants in China (for hand-washing), or - in great abundance - in some Chinese mosque's "ablution blocks". But I doubt there is any connection here... Some Russian folk tales, I think, mention the character using some kind of magic water when fighting Zmey Gorynych (a multi-headed dragon of sorts) to prevent the creature from growing new heads to replace the ones he's lost. But I doubt this is relevant here either.
P.S. Thanks to Wikipedia user Cam, here's an article that devotes several pages to the discussion of exactly this motif: Suzanne Macalister, "From the hero with a thousand names to Perseus, Bellerophon, Demetrius, George -- as Media", published in the ''Journal of the Hellenic Diaspora''.
According to her, there is no official explanation (i.e., an inscription on the icons themselves, or a discussion in canonical literature). The most common word-of-mouth explanation is that the boy is a person rescued by the saint from the Turks (or, with an earlier Byzantine reference, from the Bulgarians, with whom the Byzantines often fought). The "teapot" becomes a winecup in one iconographic version, where the boy captured by Turks had been serving a cup of wine to his master at the moment when St George responded to the boy's mother's prayer and miraculously rescued the captive.
In another version it is not a boy but a princess, who got a ride with St George to get water from a well that had been invaded by the dragon (whom the saint was to slay).
Yet another legend, from Crete, says that the small person worked at a coffee shop, where the saint was drinking coffee at the time when the message came that the dragon was located and needs to be slain; ever-helpful, the coffee guy accompanied his customer to the battle, with the coffee pot ready for action.
There are other versions as well.