My mother, according to her documents, only has a residence permit in Ukraine, for an indefinite period...?

According to Article 7 of the Law of Ukraine "On Citizenship of Ukraine", a person is a citizen of Ukraine if (any of the points):

• parents or one of the parents at the time of her birth were citizens of Ukraine;

• was born on the territory of Ukraine from stateless persons who legally reside on the territory of Ukraine;

• was born outside of Ukraine to stateless persons who permanently reside on the territory of Ukraine on legal grounds, and did not acquire the citizenship of another state by birth;

• was born on the territory of Ukraine from foreigners who legally reside on the territory of Ukraine, and did not acquire the citizenship of either of the parents by birth;

• was born on the territory of Ukraine, one of whose parents was granted refugee status in Ukraine or asylum in Ukraine, and did not acquire the citizenship of either parent by birth or acquired the citizenship of the parent who was granted refugee status in Ukraine or asylum in Ukraine by birth;

• was born on the territory of Ukraine from a foreigner and a stateless person who legally resides on the territory of Ukraine, and did not acquire the citizenship of the parent who is a foreigner by birth;

• is a newborn child found on the territory of Ukraine, both of whose parents are unknown (found)

A person who has the right to acquire Ukrainian citizenship by birth is a citizen of Ukraine from the moment of birth.

So, don't worry, there are enough reasons for your son to receive Ukrainian citizenship.