Thank you for your query.
By the description provided by you it does not appear to be of any significance as a slight variation can be observed on the two sides of the waist. However, it is advisable to get it examined by your doctor to rule out any problem in the abdomen. Some swellings such as those related to the kidneys may appear as a bulge on one side. If required the doctor may ask for an ultrasound
of the whole abdomen.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.