Please don't blame your son too much. He is still very young (2 years old,
right?).
The books we have borrowed from the libraries often have marks here and
there, and torn pages here and there. So you can see that it is a very
common problem.
If the book is unusable now, then you may talk to the library and pay the
fine. However, if people can still read the book, then I don't think you
need to worry about it.
From now on, you may need to prepare a spare drawing pad close-by, when your
son reads library books. When he is inspired to do some writing or drawing,
you quickly supply the drawing pad, and ask him to only draw on the pad,
instead of the library books. By and by, he will get the idea.
Good luck! :-)