Ruin Explorer
Legend
I wouldn't allow it, because you'd be making a rod for your own back when he later decides to produce acid factories and so forth.
I'd rule that the acid gas he produces is volatile and evaporates into stable compounds upon leaving his body. That's why the acid breath doesn't have an 'ongoing acid damage' component if additional justification is needed.
That's my recommendation.
I'd tend to agree. Even ignoring the potential game-balance stuff (notice the player seems to be trying to be helpful on this), it seems like the acid must evaporate/break down within seconds.
I mean, I can see uses for acid-breath, like blowing into a non-magical iron lock or delicate mechanism to potentially wreck it (you could certainly destroy a very large amount of textiles or food with it), but storing it? Seems far-fetched given how it appears to operate, even if the player is willing to be balanced with it.