And at 16, the son would be old enough to have input into the decision. It has been quoted here that he didn't like to spend much time with his father.
The conflict between the son and father would have been in 2023. How the son feels today, in 2025, is another matter. Two years is an eon at that age.
Plus, in 2023 the children were living with Erin and listening to what Erin was telling them about their dad. I assume for the last two years they've been living with Simon. Considering everything that has happened in the interim, the son may very well have reassessed what he previously thought to be true.