There is some amount of protection until the adversary discovers the new number. But since they've already compromised his phone they likely have his dad's numb

There is some amount of protection until the adversary discovers the new number. But since they've already compromised his phone they likely have his dad's number and can compromise that phone to find him again. It's dystopian. If he's running iOS he can also enabled Lockdown Mode on the new phone to block most types of attacks.