A couple questions...
What was his birth length, and what is his current length? I know the weight seems high, but how is it relative to his height? My dh's friend's son weighed 19lbs at 5mo but he's really tall, and his parents are 6' and 6'6(5?)" and of a healthy weight to match.
Is he otherwise an active child, playing and moving, or does he do more quiet entertainment(including tv time, if any)? Does he sit, crawl, stand, walk, etc? Is he meeting those developmental milestones within a reasonable time frame?
Where is his weight distributed? Is it carried around his middle or is it distributed throughout his frame? Belly fat is a key sign of overweight/obesity but if he's not carrying it there, might he be carrying it in large muscle mass? If he is carrying it in large muscle mass, i'd be watching his motor skill development just in case he's got something wrong with his muscles.
Can the parents afford to take him to be seen by an endocrinologist, preferably one that specializes in pediatric diagnosis? I do agree that he should be looked at to rule out an underlying problem. At least in Canada, my experience with regular pediatricians is that they don't know much about less common disorders. Otherwise, if he looks & acts healthy why assume that he's not?