I think American Apparel. It runs small (size up), but it is also quite slim. Mini Boden runs pretty slim and so does Gap, to a lesser extent. My son is 25th in weight, 50th in height and the one place I am steering clear of for him is Hanna Andersson for sure.
For my daughter (tall and slim) I got her plain cotton Pajama tops from Lands End that were a larger size and a bit tight and that worked pretty well for a year.
Danielle, Nick, Gigi ('08) and William ('11)