How to handle missing attribute's value with ID3 algorithm?
One approach is to use imputation to estimate a value for the missing attribute, based on other attributes.

See also https://en.wikipedia.org/wiki/Missing_data#Techniques_of_dealing_with_missing_data and https://stats.stackexchange.com/questions/tagged/data-imputation and https://stats.stackexchange.com/questions/tagged/missing-data
and https://stats.stackexchange.com/q/208845/2921.