1700 calories a day is low by half if he wants to gain weight. I think most of us have struggled with our sons at some point needing to gain weight. Best advice we got from a nutritionist was eat by the clock and cut out snacks. 4 meals a day at regular intervals with a shake as a supplement at some point. Grazing is bad as it prevents them from putting a big meal on board at meal time. My son's 2nd period teacher did a double take when he pulled out a thermos full of scrambled eggs in the middle of class the first time. She thought is was cool when he told her why.
Best thing we came up with on our own was put more of the foods they pound on their plate. My wife is a super healthy food junky. She would cook a new balanced meal every night. Unfortunately it wasn't always things the kids like and they would only eat enough to get away from the table. I finally convinced her to cook big potions of things like spegetti and meatballs early in the week and serve it to them 2 or 3 times through the week while we eat chopped salads.