Today both my husband and his ex share custody 50/50. Mom wants to move 2 hours away and "wants to take her sone with him". My concern is that mom will 'win'. How does a father prevent this from happening?

Dad in same home and job for the 5 years since they've been divorced. Mom moves around residency and jobs. Both are involved. Seeking advice.

Thanks.

Heaven only knows how they decide, personal feelings, who they think the children would be best with, and so on.

Stability is a big key a lot of times. Is the son attending school in your area already ? How old is the child and what section of schooling are they in ? Primary middle high school.
All of that can go into figuring out where the child should live but as Fr_Chuck stated nothing is a slam dunk.