Your son is below the percentile for his height and weight for his age, however, it maybe genetic as you stated. If the family is of small statue then there is high probability that the children would be small too. If he is eating not that well, suggest giving him Pedisure or Carnation Instant Breakfast in place of his milk for extra calories and protein. Thank you for your question.
My son has always been on the smaller side. He is also 18 months old and weighs 20 1/2 pounds and is 31 3/4 in tall. My husband was a small baby. He was only 28 in and 18 pounds when he was a year. My docter said that he is perfectly fine and as long as he is reaching all his milestones and is active. I feel the same way you do about how everyones elses kids are twicw his size and the same age. My husband is now 5'9 and perfectly healthy so I would not worry about giving an 18 month old growth hormones I think those dr"s are ridiculous. Give the kid a chance to grow. Some kids dont reach their full height until they are teenagers.