Child Battleground

My soon to be departing housemate, S1, is going to court tomorrow. No, he's not a defendant. His ex-girlfriend, the mother of his child, has gone to Australia, taking his son with her. It's unclear whether she's moved there, or is only visiting. Her family claim she is just visiting, but S1 seems to think that she has moved there, permanently. She has custody of the child and has been the boy's primary carer. S1 is gleefully looking forward to using the British legal system to punish the woman for not asking him first, before she went to Australia with her son.
I wonder at the wisdom behind the step he's about to undertake. He admits that his ex has done a good job of raising the boy, so far. S1 says that, if she had asked him first, he'd probably have agreed to her taking him there. He is not seeking custody and admits there are factors about his situation which would prevent him gaining custody. He seems to expect that his son will be put into care as a result of his legal action. What I don't understand is why he'd want that outcome? Why take the boy away from his mother, when she's doing a good job raising him? Why would he want his son in foster care, just to enable him to vindictively score some point with his ex-girlfriend?
Sadly, he's been out most of the weekend and I don't feel close enough with him to have such a sensitive discussion with him. From what he has told me about it, I think it's a big mistake initiating this legal action. He seems completely focused on "teaching her a lesson" and not on what is best for his son. The boy is only about two, now. I would certainly prefer any son of mine be raised by his biological mother, if, as is the case with S1, I was happy about the way she is raising him, than that he be given to foster carers to raise. I wonder if S1's pride is getting in the way of doing the best thing.