D.W.
If he is having tummy trouble now cereal could very well exacerbate that. I think 4 months is when they might recommend it for formula fed babies but 6 months is really ideal as baby still has an "open gut" at this age:
http://www.wholesomebabyfood.com/early-solid-food-feeding...
You also have to look for physical signs of readiness-no tongue thrust, can hold head up well, etc.
But cereal won't and shouldn't make up for the formula that you think he might be missing out on. Breastmilk or formula is primary nutrition for the first year. Solids are for practice and to supplement the breastmilk/formula.