You and your wife are a partnership! If you and her can not come to a happy middle ground then poly will never work for you and her.
I personally like to live on the wild side dive right in and figure things out
in the moment. That can sometimes work for me and other time not work and only give me a "learn from my mistakes" experience.
What you have to understand is that poly is really big on communication and respect for partners feelings. She is only thinking about herself "she doesnt want to do that" "She says it would be unfair to him" all this shows that she is not considering your feelings. You asked about veto power and she said no...but it seems like her words,wants and desires are more on the dominate rather then the equal side.
Sounds like you are willing to compromise and "TRY" as long as your feeling are being heard and common sense is being used IE you don't move in with someone before you have dated them that is ass backwards.
I would suggest you and your wife help him find a place to stay and go hang out in a platonic fashion before you jump to moving in. I wouldnt be surprised if this guy annoys the crap out of your wife after hanging out. Its one thing to understand and deal with a person from a distance but you are working in a entirely different solar system when you are dealing in person or living with someone.
bottom line "Moving a person your house without knowing them BAD IDEA" As a man I don't want people I don't know or trust around my kids nor my wife obviously if this is not a thought that has crossed her mind then she is crazy or like mono said "Thinking with her Vagina"
If this guy was still living with his mom then this probably means your wife will be replacing his mother. Cooking cleaning laundry paying bills ...etc he needs to be on his own an learn to do this things before a relationship can happen with anyone for that matter.....