Quoting ☮Hippie Jesus☮:" I ended up being able to get him asleep and content for long enough to jump in the shower with the baby ... [snip!] ... keeps pulling away from the breast when he's nursing... but will eventually settle down long enough to eat for a few minutes."
Does he ever sputter at the breast when he pulls away? Or does he gulp a lot when he's nursing? You could have a wicked fast let down, which could be causing his problems. Forceful let down tends to cause a lot of spit up and gas. It would also explain why he settles down after a little bit because he's getting past your let down. Try hand expressing or pumping a little bit to get the milk flowing before you put him to the breast and see if that helps. http://kellymom.com/bf/got-milk/supply-worries/fast-letdown/
I can't remember exactly when I started co-showering with the boys, but it was probably out of desperation .. lol.. Anyway.. I normally hold them against my chest while I wash with the other hand/arm. Co-baths are fine too, but they take longer and with 3 kids, time is important.
When I co shower with ds I just sit down in the tub & key him sit in my lap. I have a fuzzy blanket on the floor with his towel so whenwe're done I can just lay him down & wrap him up without standing. Then I rinse off & get out