Is it too soon for 1st foods?

Our second son is 3 1/2 months old. We had to start him on cereal in the bottle at 4 weeks because the formula alone went "straight through." He watches his older brother eat, and he's starting to show some interest in "real" food. We've mixed his formula THICK with cereal and, while he seems satisfied, he still wants a bite of whatever we have (he has no teeth, but I don't think he knows this might be a problem). We've given him small tastes of food here and there, but if he sees any of us eat something he drools and kneads his tongue like he wants a bite. Is it too soon to start him on some mild purees as long as his formula is still the bulk of his diet? Our oldest had no interest in "real" food until he was much older- I don't think he wasn't even trying cereal until 4 1/2 to 5 months, so this is new to us. How soon is too soon?