Meanings
because of him, for his sake
Das ist alles seinetwegen passiert.All of this happened because of him.
as far as he is concerned (he doesn't mind if); for all he cares
Seinetwegen kannst du mitkommen.As far as he is concerned, you can come with us.
Examples
GermanEnglish
Warum machst du dir seinetwegen solche Sorgen?Why are you so worried about him?
Hat Tom Sie je gebeten, seinetwegen zu lügen?Has Tom ever asked you to lie for him?