While I was on LDN and 5-ASA, my fistula healed. Whether the LDN played a role I can't say for sure. I "think" that the LDN gave me a normal functioning immune system which resulted in it healing... but I've no scientific evidence (or training) to back up my theory.
I'm on Remicade (but it is not working so far)... and it is known to suppress the immune response... so I wouldn't expect typical healing to take place given it's properties. (but my understanding is pretty limited).. I doubt that adding LDN would benefit either.. as it merely causes a 'rebound' effect... but with no immune system to heal the fistula.. I fail to see it resulting in positive impact. I think your best shot may be to listen to your dr..
Yeah, Imuran (Azathioprine) is a tricky drug. I was on it, but it wasn't a good experience.
But, that was me... an exception to the rule. Your doctor wouldn't suggest it without weighing the benefits/risks. I'm guessing this isn't his first go round treating fistulas.