"Ich werde ihn Mittagessen nennen."
I think the problem here is once again that the German 'ihn' can refer to any masculine noun like an apple ('Der Apfel'). So you can say 'Ich habe nur einen Apfel. Ich werde ihn Mittagessen nennen' - 'I only have an apple. I'll call it lunch'. Unfortunately, duo seems to mechanically translate 'er' to 'he' and 'ihn' to 'him' which often doesn't make any sense in English. To justify duo's translation we'd have to talk about fables or cannibals here.