Relocation is probably their biggest stress of all. So when you first had him, he no doubt had Relocation Stress. That manifests as refusal to eat, moping around not doing his normal activities. I know that Ignatious had a very bad break in the first half of his life, but even a change for the better freaks them out.
Then after three weeks to three months, their normal personalities slowly or quickly re-emerge, and then you get to see what your ig is really like.
I'm not saying that this is the only explanation for his sudden personality change. But is is a valid one that new owners should be pre-aware of.