Fridge Brilliance: John's anger and disgust at Ellen and Chris' decision to leave Tam and Kim in Bangkok with monetary support, which they act as if it's some grand, magnanimous gesture that's best for all involved but in truth is about maintaining their comfortable life isn't just on moral grounds. We know that John has been working with an organization to help "Bui Doi" children and knows how badly these children have it—he explicitly warns Chris about this—and is outraged that Chris is willing to be oblivious to this and leave his son in that situation.
Fridge Horror:Following the Vietnam War, we learn that Chris is a classic example of a Shell-Shocked Veteran—nightmares, troubled marriage, plagued with guilt over inadvertently abandoning Kim etc. What's his life going to be like now that Kim's killed herself, mostly to force him to take their son to America with him, but also because she's despondent at having lost him?
Same goes for the boy: how well is he going to grow up with a father who might resent him for the aforementioned reasons, and a stepmother who didn't really want him? Really, one of the biggest tragedies of the story is that Kim's death was utterly unnecessary. If she had not reacted so poorly to Ellen, she wouldn't have gotten Chris back, but she would probably have scored a green card and maybe a reasonably comfortable life in the US.
There's also the fact that Tam is going to grow up in America at a time when there was plently of discrimination against mixed-race people, and the fact that his step-mother and father are both white might not help. He potentially could end up as bitter and disillustioned as the Engineer with all the trauma in Tam's life.