As recommended by his pediatrician, i gave my 3 week son rice cereal for 1-2 days to help his reflux. i heard it can cause issues. what harm did i do?

Thickener: The american academy of pediatrics recommends solids at six months. However, in the treatment of reflux, a spoonful or two is used to thicken feeding. In your case, you did your baby, no harm, since he was on the rice cereal for such a short time.

No harm: This is common for kids with reflux. Many times it helps. Some argue that it has no nutritional value and is the equivalent of giving sugar. I believe that is overstated. It is gluten free and is unlikely to be associated with allergies. If you give breast milk and eat a candy bar, they'll get that too :).

Rice: It depends on how you are giving it - if you are giving it on a spoon then he could choke on it. If you are mixing it into formula or breast milk then it can cause constipation if you use too much (by constipation i mean hard stools not infrequent, soft stools). Other then that not much harm. Many formulas have the rice already mixed it to thicken and decrease reflux.