Fussy Eater

My infant son is a mom’s nightmare when it comes to feeding. He has been slightly under weight. He does not latch to my breast for more than two minutes. I told the pediatrician about this habit and he said that I haven’t taught my son the right way to catch the nipple. That’s why he doesn’t get full amount of milk and feels hungry. My son is very active and has a sunny nature. He likes to smile to everybody who cares to look at him. This endears him to every body.
I have tried everything from pulses to potato to ready made baby food, but he eats like a bird. How to make him eat more?