The preposition für requires the accusative case, thus meinen Sohn is in the accusative case.
Sohn is masculine, thus mein takes the form meinen for masculine accusative singular.
mein is a possessive determiner (possessive adjective) -- the possession is shown by using this word, not with the genitive case. The word mein takes an ending corresponding to the case of the noun, i.e. in this case accusative.
The genitive meines Sohnes would be used if the noun Sohn were in the genitive case, i.e. "of my son".