Having figured the bottom half out, I went through a couple of tests of the arms and head before landing on the final shapes. I am happy enough with this to move forward into final fabric, though I still need to give him feet.
I was much happier with the belly effect using gussets, but found it hard to find the right balance and push the legs forward.
Moving to stretch fabric, I adjusted for the stretch and spent several days trying to make the belly more prominent, but was ultimately unhappy with the effect. I decided to move to a new approach.
I refined the shapes that I had on paper, and then tested them on their own with non stretch fabric. I made a few on the fly edits, which I translated back to paper.
I built a model of the approximate shape using foil and masking tape, and then I fit non stretch fabric over it to get an approximate shape.