I have no suggestions.. just more questions to add to your list .
The vet found crystals in the urine, did they check for bladder stones? When they put him on meds for the crystals(?) did they do a sterile culture and determined which antibiotic was the correct one?
I think (hope someone confirms this) black tar diarrhea indicates blood in the stool?
As for his weight; 77 lbs may really be his good weight. I know with mine, going by his height and length you would think he would be quite a few lbs more - he consistently stays at 70 lbs now.