"The boy is calculating" sounds more to me like "Drengen er udspekuleret" i.e. it is a conniving, emotionless, possibly malicious boy, with his sights set on some goal...
How the hell should I see the difference between regner-raining and regner-calculating? o_O
If the boy is raining, I'd be more worried about some kind of virulent outbreak than distinguishing the two :)
But... A few years ago in Britain it was raining men, alleluiah, so why not boys? Those ladies are cougars now...
Good point :) But even then you have syntax to guide you. Det regner mænd = it is raining men. Mænd regner = men are calculating.
Actually one can quite easily. If we are talking about rain it will always say "DET regner" (like "IT is raining" in English). If there is no "Det" then we are talking calculations, not rain.
The answer you mention would be 100% grammatically correct, but does not sound very natural. One would typically answer with "Han/hun regner" or "barnet regner" if we don't happen to know the gender.
My guess is that 'regner' is for performing calculations and 'tæller' for checking/getting the number of something. It's only my guess based on other sentences in this skill. Plus I assume that 'tæller' has something to do with 'tal'.