Another word for "ignorance," in the context of "ignorance of the law is no excuse"

Solution 1:

The word that your son was thinking of is nescience (and is pronounced as your son remembers).

Definition:

nescience noun [ U ] /ˈnes.i.əns/ formal
​
the state of not knowing

Cambridge Dictionary

Etymology:

nescience (n.)

"ignorance," 1610s, from Late Latin nescientia, from nesciens (see nescient).

Online Etymology Dictionary

Usage:

Google Ngrams shows that by far the greatest usage of this word relates to Buddhist philosophy:

An extreme wing of Advaita holds the view that there is only one nescience and that nescience reflects Brahman and as soon as that reflected soul attains release there is the destruction of the nescience.

New Indian Antiquary, Volume 2, Page 93, Karnatak Publishing House, 1940

However, the usage that you posit is not really valid. "Ignorance of the law" is a fixed expression, and nescience does not appear to be a legal term.