I agree with Kris. The pooping issue is likely how Walter is telling you he isn't feeling well. If we can make some progress on getting his BG under control, it will hopefully resolve. And as Kris said, the best way to do that is going to be home testing so you can make proper dose adjustments based on the more accurate numbers you get at home.
My hunch is that you are home testing since you mentioned his BG this morning before breakfast. If that's right, let us know and we can help you get a spreadsheet going (we have one already made that you can use) and start to figure out how to get him feeling better.
As for the soft-serve, I would also suggest getting him on a probiotic. A lot of folks around here use s. boulardii. You just mix a little into food and most cats don't even notice it's there. You can also try slippery elm bark for a quicker fix. If you're interested in trying either of those, I can direct you to some more info.
I'm so sorry to hear that Walter (and you!) are having a hard time. This all takes a little time, but he really can get better. Hang in there and keep posting with your thoughts and questions. We've all been there and we're here to help.