What precisely is "Celui" grammatically? I'm trying to find it in Wiktionary and coming up blank.
It's the genitive/dative form of "cel". See the declension tables here.
It would be nice if Duo would include this info in the word definition. It would be really useful for inflected languages. On the other hand what's that phrase?
"Everything is wonderful and nickbii isn't happy."
This would make more sense as 'son of the best father'
son would also be correct. in romanian son would be 'fiu', 'fiul' articulated