Black books, black books, black books.......get him some A4 black sketch books and a decent technical pencil and encourage him to copy straight out of other peoples books, and trace trace trace.
Good artists borrow great artists steal - pablo picasso
If he has an opportunity to apprentice, thats half the battle won. Now he just has to play about with developing styles till he has a clue what direction he wants his work to go in, as at some point he will have to pick what he bag is....all good tattoo artists are known for having there "thing" be it Portrait, Irezumi, Black-and-gray, vintage 3 colour, Typographic and so one.
He will never make good money being a jack of all trades, and master of none.
A small $100 investment in a airbrush and compressor would go a long way too, that way he get used to maintaing machinery and will help dramatically with his understanding of colour, shading and hand style for lettering work.