I always knew the word pippin to be a type of apple, but then came across a reference to ‘the expression of his pippin face’. According to the OED, it was originally a derogatory term referring to a ‘young, foolish, or naive person’.
However, an American usage is apparently more current and more positive, meaning ‘an excellent, pleasing, or beautiful person or thing’. Examples of this happier meaning include the Sunday Times: ‘The Queen opened the glittering new galleries and made a pippin of a speech’, or an OED example: ‘But she’s a pippin as sure as you’re born’.
It also seems to carry an element of endearment, so I take ‘his pippin face’ to be something like ‘sweet’, or ‘dear’, or ‘darling’.
Source: OED online