I remember (around 1980) visiting the library and reading this wonderful book about a little boy with a striped sweater that was amorphic. He could store all sorts of things in it. He had adventures with another character, maybe his uncle, or some other male relative. I recall big machines, like vehicles with all sorts of ‘chitty chitty bang bang’ style appendages all over them.
The boy might have had a pet…but that’s me just imagining things, I think.
I’ve been searching for anything like this for years to no avail.
I think this is the same book that I am looking for! All of the things you mentioned adds up with my menories of it. Additionally I think the boy’s uncle was an inventor and some sort of a villain was after his inventions. In the end they had to escaoe the bad guy and did so by stacking bricks (or maybe wood blocks) to build a staircase. They removed the bottom brick and placed it at the top and in this way they took to the skies.