I don't have any answers for you but hopefully bumping it up will get it some attention
breastfeeding baby-wearing bed-sharing herb growing/using no circ'ing Mama
6-25-200611.29.20087-6-201010-3-2012 craving #5 ALREADY
It might be that his legs/knees JUST reach the edge of the fabric there, so they're pressing into the fabric when bending to make a seat. You could try putting a piece of fabric under him or even babylegs? As long as it looks like sock marks and it's not causing him discomfort, it's okay, but I can understand why you'd want to soften it a bit. As his leg grows a bit, it will become less of an issue. HTH!
My daughter had the same problem when her legs were shorter, but because it was still early spring at the time, I was able to pull her pant legs down to cushion her. They were riding up when I put her in the carrier or she was wriggling around. If he's bare-legged, I agree with the other posters that a soft cloth would work well until his legs get long enough that the seat fabric won't poke him.
Naturopathic physician, licensed acupuncturist, writer, avid commuting cyclist and community-theater performer; joyful mother of Kathryn (adopted summer 2011) and Luthien (born 11.30.12), and guardian of seven feline ascended masters!