Last night was feeding night. He pooped again, just before feeding, and the poop was brown this time. Which is less worrying.
Since I got him, he has noticeably grown in girth and in length. He was as big around as the mouse fuzzie all the way up to the second feeding (12 days after I got him).
Last night's feeding, I gave him two mouse fuzzies. I held it up to his middle, and the mouse was smaller this time than his middle... And lacking any means at that time of night to run to a store and get a frozen rat/mouse big enough for him at that point, and also because what you said was worrying me that he was still not being fed enough, I made a quick decision to thaw and feed him two... I really hope this wont be detrimental for him.