My Jake's ribs showed when he was a toddler.  So did my daughter's.  I asked the pediatrician about it because I was concerned, and he said it wasn't anything to worry about.  Some kids just don't lay down a lot of body fat and are naturally lean.  As long as he's not showing other symptoms of malnutrition (sunken eyes, dark circles under the eyes, swollen belly, sores at the corners of his mouth, pale mucous membranes (which isn't necessarily a sign of malnutrition but can signify anemia)..) then it's not really much to worry about.

My kids were all lean... can see the ribs if they stretch just right, although not constantly. I was always tiny when younger too... so I never thought much about it. I think you've gotten great advice here

'Is it OK to see Someone's Ribs?'
No. At the very least there should be a layer of skin in the way ...

Totally depends on the kid, my 17 month old and one of my 4 yr olds are "ribby" the other 4 yr old isn't. Matter of fact, the smaller of my twins still has the same waist size at a 24 month old, and she is tall, I hate buying her cloths.. *L*.

My cousin went on an all marshmellow diet when he was 2, he was ribby also, but I'll be darned if anyone could get him to eat anything else! LOL.