I'm so glad to hear this, it has been a long and stressful road for the two of you. I'm sure he'll start gaining weight soon as he eats more and more, but the fecal is an excellent idea, just to make sure he doesn't have a high parasite count that could be preventing him from gaining.
If he does end up having parasites or worms and needs to take an antiparasitic or antibiotic for coccidia, I'd make sure that he gets probiotics as well, just to make sure he doesn't stop eating again because the meds are upsetting his tummy.