He looks fine. It's normal to see some sign of ribs when they are moving around especially when they are as young as he is. Chams don't store fat along their sides. A better way to judge his body condition is to look at the base of his tail when he's full grown. His tail should be smoothly oval from top to bottom, not completely round (too fat) or show the tail tendons or vertebrae (too thin). His casque should be rounded, not sunken.