I need advice

I have been given a 6 week old Weimaraner puppy his mommy stopped feeding the 10 pups at 2 weeks old. I have had Silver for two weeks now making him 8 weeks old today. I have been feeding him a weetbix in the morning and a half boiled chicken breast in the afternoon and he gets the other half at night and he eats pedigree puppy chunks throughout the day. My problem is his hip bones and rib bones are still very prominent and he looks way too thin. He has had his parvo shot and been dewormed at 6 weeks old. Last night I gave him some semi cooked chicken liver. Am I on the right track or am I doing something wrong after two weeks he still hasn't picked up weight and I am concerned. He is a grey and all his brothers and sisters are chocolate brown and they are picking up weight and looking good.
Need some advice please