Is my baby growing too fast?

my son was born at 8 pounds 14 ounces, he only lost 3 ounces by the time he was discharged form the hospital. He is almost four months old and now weighs 19 pounds, he eats 6 oz of formula every 3 hours. Is this unhealthy or abnormal? is there anything that I should be concerned about? my pediatrition didnt tell me anything other than for a baby his size there is no problem feeding him that much as long as he keeps it all down... he is also very advanced development wise, he can sit up practically unsupported, roll over from his back to tummy and from tummy to back, he has amazing head control, he can reach for and grab at toys as well as grab his pacifier and he has the coordination to place objects in his mouth and pass from one hand to the other. he also is constantly "talking" and laughing, and playing games with us. And he also already has his two front bottom teeth in already. Someone told me that babies are not supposed to be that well developed for being that young, which makes me really worried. Someone else told me that those are all signs of autism... Should I be worried, or are these people just really un-educated (I'm a first time mother, so I have noooo clue what to believe and what not to!)