I can't recommend actual language classes enough. I was in Norwegian classes for two years - immersive classes for adults, taught by native speakers, about 15 hours a week in class. All in Norwegian, from the beginning. It was so wonderful getting real-time corrections and as an added bonus, a lot of culture/civics was worked into the lessons as well, in no small part because vocabulary centered around fairly practical subjects. If I ever need to learn another language, I hope I can have something similar.

Nevøen min lærer norsk fordi han vil bo i Norge, og I want to suggest some more resources or options to him.

We used the books "På vei" and "Stein på stein" the first year. The second year I took "helsenorsk" and we used different materials, but for the general learning we mostly used "Norsk gramatikk", which, as you guess, concentrates on about a B1-B2 level of grammar. The first year books come with optional audio, which we used in class, and both have workbooks. I used one other book, but I don't remember the name. If he's determined, he can work through the books but understanding folks is going to be difficult (some dialects are still difficult for me).

NTNU offers a free online beginning course: They also offer classes, but those are filled with current students first.

I'll add that the level of Norwegian he'll need depends on what he'll be doing here: If he happens to be getting married and is from outside the EU/Schengen area, he'll probably have classes. If he goes to university, he'll need Norwegian for a bachelor degree, but a lot of classes above that are in English. Work really depends on the field, but even in fields with lots of English in the professional realm, knowing some norwegian will help lots.