Weight gain

Image a 37 lb skye terrier. He just became a champion. He ilacks 5 points to becoming a grand champion. He is a erratic eater and always needs weight. 37 lbs is a good weight for him but sometimes he eats well and then quits and loses his weight and becomes much to thin. His erratic eating effects his coat as well but he is in perfect health otherwise. I have tried all the tricks to get him to eat and put on or keep his weight. I have been breeding and showing skies for 40 years and at my wits end. I am sure he will eat the Statin balls and I am willing to give it a try. He gets 2 cups a day of dry food a day.  How do I calculate how much to give him a day.. Sometimes I give him some canned food or chicken  or beef on the dry kibble. Sometimes he eats the goodies and leaves the kibble. He will never eat the canned duck and potato. Please help !!!!!!!???
Barbara Lubliner
Barbaralubliner6@gmail.com