Screenshot of the uncle would be helpful. I have some guesses, but it will be easier that way.
He could be bishop for example, that way he may not inherit.
Though, if he is not bishop, just give him some county and duchy - that way you will be able to choose him.
You were right.. I granted him a county and he appeared on the nomination list!
Strange thing is, plenty of people who appear on the list are not landed.. eg my daughters. So what exactly are the criteria to get on the list?