I don't really know. Pairing him with someone with no mutations at all would be optimal with regards to figuring out what mutations he carry and how they are passed on. But finding a bird with no mutations will be pretty hard, taking into consideration that the mutations can be carried invisibly.
If the goal is to get more like him, pairing him with the female that's most related to him would be best.
In general I'm not a fan of inbreeding but I'll have to admit I have a male I don't know the mutation of as well, and if it hadn't been for the fact that he started pecking his daughter, he would be paired with her now ^^ I'll probably try introducing them again in a new invironment in a few weeks. None of his chicks have shown any mutations, so I'm pretty sure whatever he is, is recessive. So to get more like him, I have to pair him with his own offspring (or pair the offspring together, but I was stupid and sold all the males and apparently he (or the hen he is with?) has low fertility now, so getting more chicks isn't easy).