How to handle missing continuous attribute values in ID3 (Iterative Dichotomiser 3)?
Take a look at the C4.5 algorithm.  C4.5 is a successor to ID3.  It is more complex.  However, Wikipedia lists as one of its advantages over ID3 that C4.5 can handle data with missing attribute values.  So, you might take a look at how C4.5 handles missing attribute values -- or even simply use C4.5 rather than ID3.