Is my grandchild underweight

My grandson is 4 weeks old and at birth weighted 7 lbs 1 oz and 21 inches long. He is breast feed and lost to 6 blbs 9 oz the frist week and half, but supposley is up to around 7lbs 12 oz and 23 inches long. Each picture I see of him he looks to be getting skinnier and skinner, but his parents do not feel there is anything to be alarmed over and refuses to consult his Dr until his scheduled visit next week...The last picture I received from my son the child skin was very wrinkled and hang from his little legs and his ribs are showing...The picture of him was in his bed with just a diaper on...I was mortified and very concerned he is not getting enought to eat..He does cry often and I am being old that he cries becasue he is MAD....but a 4 week old baby does not scream for no reason..Please help me understrand if my concerns or valid or am I over reacting

[Renee Radenberg]

If the measurements are correct, his weight is appropriately between 5-10th percentile and his height is between the 90-95th percentile for his age. He is tall for his weight so he appears thinner than he is. If he is eating and gaining weight at a steady rate, then he will grow.