My MIL opinion

I'm a FTM and I depend on the doctor advice n opinion I know some mom's already been through it but that doesn't mean everything they say is true

My son is 1 month old and has digestive tract problems mainly gas an constipation... I give gas drops n Dark Karo as instructed by the doctor even though they aren't helping an my poor baby still has to strain to poop with no success n it's been 4 days now but he's still taking formula every 3hours he pee's just fine just no pooping... My mother in law insist that I give him cereal... he already has GI problems the doctor said his tummy is very sensitive I don't wanna give him anything that will mess with his stomach... I never asked the doctor about cereal I just know he's to young for it... my MIL says "the doctor don't know what he doing the did it for previous babies and it works" I don't like to see him miserable I don't know what to do

My question is should I listen to her n give the cereal or no?

If not what can I do for his constipation and tips or tricks would be greatly appreciated