First of all, I don't think the issue here is really how to tell a two-year-old his dad died. The issue is how to prevent the father dying in the first place. This all sounds so passive to me. If the father is refusing to get medical help and actually wanting to die, I'm wondering if there is mental illness at play here. The wife needs to suck it up, find someone to drive both of them to a major medical facility and go from there. If he's a veteran, he likely will not end up having to pay for care at all and in any case, they will treat him and work with the family after care has begun on payments. If their income is low, they will likely qualify for state aid and it can be retroactively applied.
Another concern here is whether this dad is able to care for his son while mom is at work. If he's suffering from kidney failure it is unlikely he's in any shape to run after a two-year-old or handle an emergency. What if he lost consciousness or even died while the toddler was dependent on him for care?
I think it's great you've stepped in to look for answers, but now is the time to call in the big guns. If the wife can't get him to the hospital and he's really so ill he's facing death soon, an ambulance should be called. Speak with his Legion buddies, with the local doctors or social services. This situation is an actual emergency that doesn't need to result in a two-year-old losing his dad. Even if the dad has a fatal condition, he should be evaluated and at least given help managing pain.