Not sure if Mo considers a license plate not being visible as a moving violation or equipment type of violation. The plate could have been stolen or knocked off by road conditions, so I would think the purpose for the citation would be more to correct the problem than punish the vehicle's operator.
Not appearing in court and answering to the charges however, transforms a minor violation into failure to appear, which is a criminal matter. This obviously, resulted in the warrant for his arrest.
If he now does what he originally should have done and appears before the Judge to resolve the matter, it is anyone's guess weather or not the court will take him into custody. But it is very unlikely, unless there's more to the story than the license plate issue.
However, if your nephew does not take it upon himself to appear in court voluntarily, but instead, decides to wait until he is again stopped or questioned by police, who will inevitably discover the outstanding warrant, you can about guarantee he will be arrested and taken into custody.
Tell him to get his lazy, irresponsible a$$ to court and clear the matter up while it is no great inconvenience for him to do so. Because he will never know where or when the police will decide to run his name and when that happens, his fate will be decided for him.
Have no doubts, sooner or later, it will happen.