I kinda understand him (not that I will do the same). Like he said if he told her then she would have wanted to be with him. Even if he left behind a letter telling her of his condition and choice and went to the US without telling her then there is the possibility she might wait for him. Instead, giving the illusion that he abandoned her, she might be heartbroken for a while but may eventually get over it (remember that he didn't know she was pregnant). Maybe he wanted her to move on and live a happy life with another in case he died.
I think you also have to consider that if he had told her, she might have wanted to follow him. if she had, she would’ve had to deal with a partner in a coma, living in a new country by herself with a newborn and no support system, all while possibly not speaking the language fluently. The alternative is her taking on the mental & physical load of taking care of a newborn & a declining partner. I don’t see a good outcome, so this is most likely why he chose to not tell her even though it sucks to not be told something so important :/
I really sympathize with the dad but it was still wrong of him to rob the mom of her agency and choice in the situation. She died without knowing the truth :(