My daughter is almost 5 months old and sometimes doesn't poop for a week. My paediatrician assures me this is normal for an exclusively breastfed baby. But when my baby does seem bothered by it, I use a glycerin suppository to get things going and it works like a charm (my paediatrician said she'd do the same...she also suggested giving diluted prune juice 1 to 1). My mother, who is a nurse and used to work on Post-Partum, also said I could use a gloved pinky finger and insert it a bit to get things going (similar to the thermostat idea).
But these are solutions that have worked for my daughter from 4 months on. I'm not sure if it's OK for a 6 week old.
Good luck! I know, their poop is so worrying!