Has he gained steadily since birth, or has there been a sudden pause in his weight gain? At his check-ups, do you discuss this? What do they say? Are they measuring his head size to be sure it's growing? Our pedi told us that was more important than anything. Does he appear to be hitting all his other milestones? Is he happy and active? Have you discussed everything you are feeding him with the pediatrician? Are you feeding him on demand? I think you should be. If it's 3 meals and 2 snacks, great. But if sometimes he needs more, then give it to him. While I don't think you should reward a child with food because it creates bad habits, I think food should be offered as needed and not entirely on a schedule if a child's needs dictate otherwise.
When you say "3 meals and 2 snacks," what do you mean? Do you include breast milk or formula, or do you have him entirely on solid or pureed foods? I hope he's on a good amount of liquid nutrition (breast milk, formula or other approved supplement for infants), and not just baby food. He's still probably developing his digestive system as well as his ability to maneuver/swallow solid foods, f if he's giving up because he's frustrated or tired, that's a possible problem.
If your doctor is not worried, but you are, are you saying that you don't trust the pediatrician? If so, then switch NOW and get your child's records transferred. You must have a doctor you trust.
However, if everything seems to be fine except that number on the scale, I'd wait it out without worry. Some kids are just little always, and some are little at one point in life and then get a growth spurt later on.