Options for Travel Consent if child's parents are divorced and 1 parent won't sign.

This is a complicated one. My nephew's dad has stopped communicating with his mom (my sister) and my nephew isn't talking to him either. My nephew is 16yo. His father is being charged with a crime, and the court date is in June, our trip is in September. I was planning to get consent letters from both parents, but getting one from his dad sounds like it will be a major thing. SO I am wondering what options I have, is one parent enough? We are flying to England, France, Italy, and then we are driving to Mexico in October.