I'm not sure there is a word in the English language for that!
However, if you're unsure how to address him, you should use his first name.
This will avoid any awkwardness about his proper title.
If you want to be a bit more formal, you could call him Mr. and then whatever his last name is...
Why not ask your wife what he'd prefer to be called to avoid confusion?
The names for relationships with members of your extended family are so complicated, that I'm not surprised you weren't sure how to refer to your wife's uncle's son!
I agree with James that it would be much easier just to use the person's name, as complicated relationship titles can be a but clumsy, to say the least.
However, if you want to know exactly what relationship your wife's uncle's son is to you, then this chart should help to work it out.
From this, I'd say that your wife's uncle's son is her first cousin, making him your first cousin by marriage...
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