I’d email Dr. Phillips at http://www.marci-kids.com and/or talk to his doctor. The foul smelling poop doesn’t concern me as much as the foul smelling urine. Could he have a UTI?
The abdominal pain could be from the Mylanta. Some babies do get crampy from it.
My 6 wk old has been on it for about 3 weeks, and we’ve not noticed any change in urine or stool odor. I would also check with your doctor to rule out any other possibilities.
Mary, when Alana had a UTI her urine was pretty foul smelling (her bowel motions have always been smelly!!!), so I think I would probably run this past someone. Over here we have a 24hr maternal child and health line- not sure if you have something like that? Anyway maybe if not a family Dr or something. I always feel better when I see the Dr (who things I am a hypochondriac), find out nothing is wrong and can stop worrying. Hope you can work it out soon. Goodluck.