foster-son vs foster son

foster-son and foster son both are nouns.

Word	Noun	Adjective	Verb	Adverb
foster-son	Yes	No	No	No
foster son	Yes	No	No	No

As nouns, foster-son and foster son are synonyms defined as:

foster-son and foster son: someone who is raised as a son although not related by birth

foster-son (noun)	foster son (noun)
someone who is raised as a son although not related by birth	someone who is raised as a son although not related by birth