Firstly, I am sorry for your situation OP, it is indeed an awfully difficult one to contemplate.
However, from a DIL's perspective, my DH moved abroad many moons ago, and until he met me, was destined to return. His mother was, and still is, desperate for him to return. And while at first, this seemed understandable, the reasons for his initial departure, soon became very apparent. She really wanted him to live the life she wanted for him, not the life he wanted for himself, and to this day, I know she is resentful. She is very financially comfortable, and openly provides financial support to his sisters who remain close by, and seems to think that financial hardship here, will drive us back to her one day. His lovely father was very supportive of his decision to stay, and sent us a lovely letter which we will always cherish, before he sadly passed away. As for the decision on staying at home to look after children or not, sometimes we DILs just can't get it right. My first MIL thought less of me for returning to work, although financially at the time, I had no choice. And my current MIL, seemed to think I was fleecing her darling son, by staying at home to look after our child and not returning to work! At the end of the day, it is their choice as a family. And if your DIL senses that discord from you, I guarantee it will not help.
Having that all that OP, I really do hope you can work this through with your son x