No, he has to pay taxes on the BTC he DID convert to fiat. He does get to write off the losses of the alt-coins but since he hasn't yet realized those losses (and since they're in a different fiscal year), he's pretty well fucked.

EDIT: even if he didn't convert to fiat, the exchange of BTC for an alt-coin is a taxable event. That makes sense because otherwise you could, in theory, convert to a dollar-pegged crypto like Tether and defer taxes indefinitely.