Welcome to the Wefugees Community and thank you for reaching out to us.
When you are talking about "IDs", you are referring to residence permits right? Yes, when your fiance has a residence permit in Greece, for example because he got recognized as refugee there, the German authorities will reject his second asylum application. However, in general also rejected asylum seekers can marry in Germany. But many people struggle to provide all the required documents. Please see our blog on this topic for more details. Once you managed to marry, it will be dependent on your type of residence permit in Belgium if you can apply for a reunification or not, I assume. Unfortunately, as the platform and our experts are based in Germany, we are not that experienced about the Belgium immigration law.
I would recommend your fiance to consult a migration counselling office in Germany. I am sure there are comparable offices in Belgium, which you could visit, no? Please let us know if you need support in finding any.
All the best,