He is specific to the Black Forest biome. He has multiple possible spawning locations and once you generate one of them, he will appear at that location and only that one location.

There appears to be a high tendency that at least one of his spawns will appear only a really short distance from one of the Elder spawns, but it is not the case for all seeds.

Also pay close attention to your map and minimap because his icon will automatically appear from quite a large distance away.