I understand your concern, Joy. Growth charts can cause a lot of worry for parents. However, if your physician is not alarmed, you shouldn't be, either.
Your son has been following a consistent growth curve, so he is growing appropriately even though he is at the lower end. What you need to look at is your and his father's body types: if you are small or thin, then your son is following in your genetic footsteps. It's also important that you look at not just weight, but also length (height) and weight for length. Talk to your Pediatrician about your concerns and have him explain the entire chart to you. As for his diet, you son appears to be eating a variety of foods. Finally, keep in mind that the serving size for a 2-year-old is much less than that of an older child.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.