I'm sorry he's having such a tough time. I always have a hard time getting rid of c.diff. Is he taking saccharomyces boulardii? I think that helps a lot along with a good probiotic. The S. boulardii that I take is by the brand Jarrow. I buy it online at the Vitamin Shoppe. The best treatment for me has always been to do what they call "pulse dosing". I don't remember the exact instructions but it's something like 3 days on Vanco w/s. boulardii and acidophillus, then 2 days off the Vanco but continue w/the s. boulardii and acidophillus, keep repeating that (3 days on, 2 days off). What it's supposed to do is let the antibiotic kill the bad bacteria while you replenish with good bacteria. It ends up being a 3-4 week regimen rather than just 2 weeks on straight Vanco.
As far as Pred, I've been on it while still testing positive for c.diff. Since he's still having so much trouble, I would talk to the doctor about pulse dosing and trying pred. Good luck w/the new doctor.
Female, Age 38
Diagnosed w/c.diff 7/02 when pregnant w/1st and only child during 2nd trimester(wasn't on antibiotics)
C.diff flare within weeks of delivering baby 1/03, Diagnosed with UC 2/03
Acidophillus, Sacchromyces Boulardii, Oil of Oregano