D.C.
Hi Gina,
I'm surprised that two pediatricians have told you they think it's constipation!!!
I too was worried about my little guy who, from about 3 weeks old to now (3 months old), has only had about one poop per week!
At both his 4-week and 2-month well-baby visits the doctor told me that this was nothing to worry about. She said that babies often only poop every few days to one week and that, as long as the poop wasn't a hard ball when it finally came out (and as long as there are several wet diapers per day, which is the most important thing), then there was no need to do anything.
Sure enough, we have about 5 - 7 days between poops that are liquid "explosions" and will completely fill --- or blow out --- his diaper).
Since your baby's poop is soft, I wouldn't interfere with his normal pooping cycle with suppositories, etc. You definitely wouldn't want to give a stool softener of any kind.
By the way, my baby is always pushing and grunting, too. But it is tons of GAS that he is pushing out!! Unfortunately, the higher fiber foods make him even MORE gassy!! (I think it's a cruel irony that the more you eat healthy high-fiber foods / veggies, which are good for you and allow you to pass on great things in your breastmilk, the more your baby is gassy and uncomfortable --- and CRYING. I'm having a big problem with that right now). Oh well, such is life!
Anyway, good luck to you...I hope this helps!