If he's constipated it is possible the poo coming out will be softer. It will be softer because it goes around the hard mass that's stuck in there.
But, that's not saying he's constipated.
Is it possible that he has food allergies?
And more importantly, are you making a big deal of it in front of him? It sounds like there is some anxiety in your family (your mom) with regard to this issue and it could be stressing him out. Try to relax and not comment on...