I can't actually see but it could very well br your genetics I wouldn't worry too much unless you are in pain. If you are not in pain I suggest just work out to be healthy and look up tri-planar movement based exercises.
If you have plenty of energy, and are eating well, you are not to skinny. Eat healthy. Use MyFitnessPal.com guidelines for calories. Balance your nutrients. Do a variety of exercises that strengthen your whole body. Incorporate cargo into your routine. Your body will conform to its correct weight. Don't starve yourself. Fuel your workouts. Fuel your day. Love yourself. Love your body.
My coxic bone sticks out aswell as my spine, ribs etc. And it hurts to do some exercises
Everyone s body process differently it takes some people longer to build muscle and gain weight than others ot takes time not to mention your young and still growing I wouldn't stress so much on your weight yet untill your older