Has your veterinarian examined your new dog yet? They would be best able to assess his weight and determine if he is too thin. A fecal exam for parasites would also be a good idea. The most common nutritional problem in US dogs is obesity, and it is generally a good thing to be able to see a "waist" from above and from the side. In a dog with ideal weight, you should be able to easily feel his ribs and see a slight bit of the last rib, but the rest of the ribs or the pelvis and spine bones should not be poking out. You can see a chart of body weight conditions at the following website: http://www.purina.com/dogs/health/AdultBodyCondChart.pdf
Kimberly Coyner, DVM
I agree with Dr. Coyner's assessment...since we can't see what you are describing, it's hard for us to give you a good answer about how thin he may or may not be. He is also reaching his adolescent and teenage years, so he may be putting on a lot of growth in length and height, but not necessarily any weight yet.
What was his last weight? When was his last veterinary visit?
Also, the type of food you are feeding could factor into his appetite as well. Lower quality, cheap store brands often have lesser energy and that means the dog needs to eat more to meet his caloric needs. What are you feeding him now?