GagaJo I'm afraid I have to disagree with you. People just don't seem to 'plan' families any more, they just go ahead and get pregnant when it suits them and when they want it, like going to a shop and buying what you want, when you want, without thinking about whether or not you can really afford it. When we started to think about having a family, we decided that the best way to see if we could afford it, was for me to continue working for a year, putting every penny I earned into a savings account, then, if we managed to get by without having to dip into the savings account, we knew we'd be OK. This worked really well, not only giving us reassurance that we could afford a family, but also giving us a nice nest egg for the necessary purchases that go with having a family.
Sorry, OP, I know that that was going off subject somewhat, but as far as I see your situation, if YOU feel you can manage to continue giving your GS the support he needs for at least a bit longer, then I think that is what I would do. Surely your DH can see that stability is what your GS needs for now, even if only for a few weeks more, but I would suggest speaking to your son and DIL and making them aware that you're struggling, they can only think about making changes if they're aware that that's what is needed. As always, in my opinion, communication is key.