Ye can do, I'm still undecided as to what if anything he can breathe.
i have 2(i keep hitting 3) ideas myself and remember one from the docfiction show i saw a long time ago(fictional show meant to look like a documentary) i think theirs used hydrogen in it
both of my ideas would probably require a second stomach (oddly the second stomach would be much safer to enter for other character elements). this would not contain acid or your own digestive enzymes but a lot of bacteria/yeast/molds/honestly i am not sure which one i want here.
it would also probably have 2 entrances, but that parts not required
the dragon would need to eat grains or grassy foods in addition to their normal diet for this to work, inside the second stomach the materials would be broken down by (bacteria/yeast...) to produce methane gas or some kind of alcohol.
a methane dragon would spray the gas out of their mouth or nose along with a small amount of some more complex chemical to start the reaction, or maybe have nerve voltage spark, the gas would then ignite and burn what is in front of you at a very short distance
the alcohol based design would involve getting a very consternated form of alcohol in the stomach that is separated somehow(that's a minor detail, and might be stored sperate from the grain). when they go to "breath" fire they spit out the liquid like spit at a target and something sets the liquid on fire, like how a ww2 flamethrower worked(they used gas but not the point). it would work at a much greater distance and i think its better, although you would have more of a stream of fire rather then the fireball that is commonly shown
... hopefully i did the spell check right