If you can't figure out what is causing it either through elimination of new foods or old ones, try the BRAT diet - Bananas, Rice, Applesauce, and Toast. OK, for 7 month olds, mash up Banana, overcook the rice or try rice milk, and keep the grains white so the kids have a chance on digesting them.
My daughter was sensative to soy, but I had a hard time figuring it out because she was eating it, gaining weight and generally ok, but had diarrhea. We started the brat diet and eliminated the say and had her back to normal in about a week. What was confusing was the poop was not traditional diarrhea, it was just soup, not liquid with pieces in it like diarrhea caused by disease.
Good Luck