I have NEVER heard of putting baby food IN formula plus, it is not recommended to start a baby on solids until they are at least 4 months old. The strongest recommendation nowadays is 6 months. Do you mean baby cereal?? Even so, there is no reason to do that unless the doctor tells you there is. Some people will do it because they believe it will help the baby to sleep longer but my understanding has always been that this is unfounded.
Regardless, he is having trouble with his bowels for a reason...whether it is the food, a vaccination he got - did this start around the time he got his first rotateq (rotavirus) vaccine?? My youngest get diarrhea for a month after the 2nd and 3rd doses of it!! His condition had to be reported back to the manufacturer because the doctor felt very strongly that was why it happened. If so, I would highly suggest talking to the doctor about not doing the 2nd and 3rd doses. Also, we had to give our son a half a capsule of Culturel mixed in applesauce to help him along. You might want to ask the doctor about this.
Grandma is a whole other issue. Do you have anyone else who can watch him because I'll tell you what, if she won't respect your wishes about this, she won't respect them about anything...what she lets him watch, feeds him, plays with, nothing. A friend's mom just took her grandson for a flu shot because she was so worried about him and didn't trust my friend too care of it. Guess what? My friend had just taken him 2 weeks before!!! She is the grandmother, not the mother and she needs to respect your wishes and rules or she doesn't get to watch him alone. I know it may be tough because she's your mom but he is your son and the one you need to stand up for now. Seriously...don't think your mom's behavior is going to go away by itself. She's already showing you it won't. :(
Best of luck and I hope you keep us posted.