Got some mostly good news today! Ended up not doing the x-rays, since when the vet looked him over, she said he is still improving. She said the fluid that had built up in the back of his fetlock was now gone, and there was just a hard spot in the front left. She is still positive that this was a very old something (injury or bone chip) that worked loose, and between that and his age (16), it's just taking a while to heal. I'm to give him a bit of bute for the next 10 days, double up his joint supplement for the next two weeks, and ride him gently every couple of days for the next two weeks. She'll look him over again then, and re-assess. Fingers crossed that he'll mend all the way soon!