As he dont mention a wound in the letter, we can only guess.
From what he writes you can tell a few things. He says he will be sent to frontline again, so we can
assume he has already been at the front. Also he says he recovered well, so we can assume he was
either wounded or sick. 100% sure you will never now, maybe if you can find some more letters
of the same person.
One more thing, as the Feldpost was sent from the Staff, and not from a Lazarett (field hospital)
i would say its more likely that he was sick, and not wounded. But thats really only a guess, he could
have been wounded too, and after he was treated in the field hospital, he was sent back to his Staff to fully recover.