There can be a number of chest wall deformities which can give rise to protruding bone on the right rib cage .The protruding out of the lower ribs in your child may be due to a Vitamin D deficiency or calcium deficiency. For diagnosis, you can get his serum Vitamin D 3 levels, serum calcium levels and serum alkaline phosphatase levels estimated. The treatment involves adequate supplementation according to the deficiency. A consultation with a pediatric endocrinologist is recommended. If you see the ribs abnormally protruding then you may have to get an X ray of the chest done to rule out deformities which are usually seen in Vitamin D or Vitamin C deficiency. It can be due to Pectus Carinatum also called as “pigeon chest” because it causes the ribs to protrude, looking bird-like. Most of the time it is a cosmetic issue and may not lead to any future problems but in some cases it could because of abnormality in spine posture. The protrusion of the chest wall gets corrected as the child grows. At present it would be advisable to get him evaluated by his doctor so that he could follow him for the condition.
I have not encountered such thing before but if you are not comfortable, why not seek a second opinion?
Asymmetry in the rib cages sometimes occur, but if increasing, may signify an underlying pathology. I would suggest getting a detailed physical examination, followed by an imaging study eg. chest X-ray or ultrasound if there are concerns. The advice is kept general and not a substitute for a consultation. Kindly consult your doctor or paediatrician as a detailed history and examination is important to come to a diagnosis and treatment plan.