I agree with Myates, the fact that you are trying to come up with an alternative does matter and beyond that, you can't do anything else. Please don't feel bad for any decision you ultimately end up making. I think we all know, as well as yourself, that it is out of necessity, not desire or uncaring.
That said, I'm sorry he is getting worse. I don't mean this in a bad way because of course, I want your little guy to get better, but maybe it would be a blessing if he did pass on his own if indeed that is his destiny at thi time, so at least you wouldn't have to help him go.
Please let us know what happens and know that there are lots of people that support you whatever the outcome!