New York Film Academy recommend Pencil2D for animation among others. It seems to do what you want in terms of the fundamentals he will need to learn for the right price…
I’m a bit out of touch as have had the privilege of using expensive tools for years now. Does suck that the standard tools for digital art are sooo expensive but most big companies know that their futures lie with new artists and offer big student discounts or even free software for student card holders. Adobe for example discount the creative cloud (here in the UK) from £50 to £16 a month, here