What are the difference between these two sentences: 1) He's a mere boy. 2) He's merely a boy
The only contextual difference comes from one being an adjective, the other an adverb. This shifts which part of the sentence "mere" is modifying, thus shifting where the meaning is assigned.
In "He is merely a boy" the word "is" is being modified. The boy's state of being is characterized by the statement. This implies a placing a greater blame upon the boy, or that his condition is inextricable from who he is.
In "He is a mere boy" the word "boy" is being modified. "Mere" is calling attention to why being a boy in this context might matter. Although both sentences mean roughly the same thing modifying "boy" conveys a more neutral attitude towards the boy.