We had a friend in Arlington who had a very, very, very old Whippet. Our son was about four and once held out his hand to the dog and the dog just fell over.
Our friend was walking him once and a policeman stopped her and said ma'am you need to put that dog on a leash. She said "officer, this dog has to lean up against me to lift his leg..."
Sadly, he got out one day and was hit by a car. Our friend actually found some comfort when my wife said, "Well, Joanne, he died a young dog's death..."
Looks like you need to feed him a little more, I can see ribs outlined under the skin.