"For my son?"

Translation: Für meinen Sohn?

[HerrSchmidt.]

Why is it ´meinen´?

[Kris831282]

Just boosting this comment. My question too!

[MarcusCoop8]

because Accusative prepositions always trigger the accusative case. ie durch, für, gegen, ohne, um

[George440392]

Only beneficium is allowed to have this profile picture

[salgsalg]

(Is this) for my son?

"Is this" is implied.
"This" is the implied Subject which is acting upon the Direct Object "Son". So, "son" gets the accusative case. Hence, "meinen".

Hope this helps.

[JustinAlle183103]

It would be nice if Duolingo would teach us about the accusative preposition, not all of us are language experts

[8z0V]

Why is it Akkusative?

[DjoALx]

Yeah, why meinen

[MarcusCoop8]

because Accusative prepositions always trigger the accusative case. ie durch, für, gegen, ohne, um

[ra7432]

So the subject is implied, right?

[janaranjani]

Certain prepositions need to be followed by the accusative case, and are known as the accusative prepositions:

für – for
um – round, around
durch – through
gegen – against
entlang – along (usually placed after the noun, rather than before it)
bis – until
ohne – without
wider – against, contrary to something

[janaranjani]

One can remember this as FUDGEBOW