Try some lamb mince, tripe and wholemeal bread (mix bread with little water), fatty beef mince too...
I have a 16 month whippet mate and he a fuker for putting weight on, he's normally around 9 3/4kg and stand about 21" to shoulder, but he's a very picky eater - sometimes he'll eat 100% of his food then next day or two he'll only eat 50%
I was told he will eat when he is hungry... I'm just learning to accept he not a big eater and he's just a lean dog (not underweight, just looks lean)...
You could try feeding smaller meals twice a day ???