If you can't get something like draxxin or naxcel or nuflor or baytril quickly...good ol' PenG is actually pretty decent for pneumonia in kids. The dosage is:
1ml/15lbs, 2x/day, for 10-14 days *through an 18ga needle*.
Needle size is important here...seems like a stovepipe, I know, but PenG is a suspension and the particles floating around are the actual medication. Too small a needle draws up too much carrier and not enough med, weakening the dose.
PenG is cheap, non-Rx, and can be had almost anywhere that sells this type of stuff...
TSC, Southern States, feed stores, etc.
Now..you say he's gone from stopped up to "very, very liquid" diarrhea...but sounds like he's got fluid in his lungs. To me, that indicates that you're at a critical point right now. On the one hand, you don't want to flood him with fluid on account of his lungs, but on the other, you have to be concerned with dehydration. That means knowing how to identify both and go the way you think you need to go.
To check for dehydration, pinch his upper eyelid. If it snaps back when you let go, he's OK. If it tents for a second, he's getting dehydrated.
If he gets dehydrated from the scour, my suggestion here would be to avoid drenching him with electrolyte right now if at all possible.. Reason being, he's already having lung trouble and drenching -- especially a goat that's kinda 'down' a bit -- can sometimes make them cough and irritate things even worse. What I'd do if I were you is try my very hardest to get ahold of a couple of bags of Lactated Ringer's solution, an IV set, and some 20ga x 1" needles. Lactated Ringer's is prescription-only for some reason, but your vet will have it. You don't have to use it IV -- you can SQ it, which is super easy to do.
Just hook the IV set up to the bag, run the air out of the line, and put a needle under the skin over the ribs.. Let it drip...drip...drip away until he's got a nice 'saddlebag' over one side, then pull it, swap in a new needle, and do the other side. His body will absorb it rapidly.
If you can't get Ringers, then of course drenching an electrolyte solution will be your only option...and there's nothing wrong with that, per se.. It's what 99.9% of folks would do, as most don't exactly keep a stockpiles of lactated ringer's on hand (like some of us more paranoid types
).
I wish you nothing but luck. You've already done more for this little guy than most goat owners, and for that, I commend you. Keep after him, and keep us posted.