Feeding

So my son turns three months old today. Since the day he was born he has always been a lazy eater. He either falls to sleep or gets easily distracted. At his 2 moth appointment, his doctor said that he should be eating around 4-5 ounces. Right now he only eats 2- 2 1/2 ounces, sometimes three if I'm lucky, of formula every 3 hours. Has anyone else ever have this problem? He's also only around 12 pounds.