(tx) Brother's Ex Went To Mexico With Their Son And Has Now Blocked Him And Is Not Responding. What Do We Do?

She has majority custody and has filed multiple CPS claims against my brother that have been closed without findings of abuse. Last week he stopped hearing from her or their son, they didn't show to the exchange the day after Christmas and there is no response on the parenting app they use. He tried to call her but it's going straight to voicemail and texts are not going through. No one answered when he called a wellness check and his lawyer has been on vacation until next week. He found out through someone who knows his ex that she was talking about going to Mexico so he thinks she took their son there.

What can he do about this?