Thank you for your query.
What you mention about the rib cage
can be due to vitamin D deficiency. I suggest that you consult a paediatrician who will examine the child, get necessary x-rays done to confirm the diagnosis and advise vitamin D. Vitamin D deficiency can also reduce the tone in the muscles.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.
Rajiv K Khandelwal