Kids with kidney disease frequently have growth issues and are often well below the norm for growth. These issues need to be addressed at an early age and actively monitored by a pediatric endocrinologist who will work closely with your son's nephrologist. I'm surprised the nephrologist hasn't addressed this with you, in fact.
One person mentioned that her children were in the 15th percentile and that you shouldn't worry about your child's growth. You should know though that 15th percentile is still well within the bounds of normal growth. Abnormal growth is considered to occur in any child who is below the 2nd percentile for height and/or weight, or a child who is growing at a rate that is 3 standard deviations below the norm (that is, about less than 1/2" per year -- normal can span from 2" - 4"/year, depending on age and genetic predisposition).
I highly recommend you speak to your son's nephrologist about your concern. Many kids with kidney disease are good candidates for treatment with human growth hormone replacement, although some are not, especially if their eGFR is low or they are hypertensive. In the meantime, you should also checkout the Magic Foundation for Children's Growth website: http://www.magicfoundation.org
Growth hormone replacement therapy is otherwise pretty safe. The hormone is now created through recombinant technology so the risks associated with its use in the 1960's - 90's have been eliminated (it used to be harvested from the pituitary of cadavars and there was a risk of contracting certain neurological diseases that you probably really don't want to know about!) It is important to begin treatment as early as possible so that your child will grow at a normal rate and keep up with other kids his age. You do have some time to make this decision, though, unless your son also has an abnormality that would cause the epiphyses (growth plates) to close early. Normally, this doesn't occur until late puberty, around age 14 - 17.
My son's pituitary gland (the gland responsible for growth hormone production) shut down around his 9th birthday. By the time I got my husband to agree that there was a problem and take him for testing our son was 10.5 and was the size of a 3rd grader (in fifth grade. He was also obese, although he didn't eat much, had very little energy, and had grown less than 1" in 1.5 years. He spent the next six years on replacement therapy, which required him taking a shot every night (he chose to give himself the shots under my supervision). At 19, he is now a strapping 5'10". In the last year of treatment, he started becoming resistant to taking the shots because he was tired of the nightly needle stick. I then found that there is one company, Merck Serono, who makes a needle-free delivery system (check out http://www.justgrowth.com/me/en/sitemap/sections/devices/.... Using this system, my son was willing to take the hormone for an additional year, by which time his pituitary had resumed functioning at the lower level expected of a normal adult.
BTW, growth hormone is essential for a lot of necessary physiological functions, such as fat metabolism and carbohydrate usage. Kids who are deficient or whose bodies can't use it properly are at a much higher risk for diseases such as diabetes and hypercholesterolemia (high cholesterol) leading to early atherosclerosis (narrowing of the veins and arteries because of fat deposits) and early heart disease. Making the decision to give it to your son can therefore be a very important one. Also, the stuff isn't cheap. By the end of therapy, the drug cost came close to $3,000/month. Fortunately, our insurance paid for it at 100%. All of the manufacturers offer financial assistance for patients with documented need but without insurance support, however.
I'm happy to discuss this with you more, if you like. You can reply to me through Mamasource and I'll send you my email address and phone number.
Best of luck!