My WCS Murphy is also 15 months old and we have struggled to keep weight on him, he weighs in at around 13kgs at the moment. He is very ribby, but has got good muscle defination around his shoulders and legs. He is a fantastic eater and we have tried various foods to try to get weight on him. Unfortunately the only food on which he did gain weight easily also made him scratch and lick off a lot of his hair on the inside of his back legs, ( a very expensive brand I may add!!).
However we are busy trying Barking Heads at the moment which he really enjoys and he seems to be holding his weight, doesn't scratch or lick, his hair is growing back and the rest of his coat is very glossy. I suspect he will always be on the lean side, but I've decided not to worry about it, hopefully he will fill out as he gets older.