I have a Rottie X German Shepherd and he weighs 50KG. He is supposed to lose 5kg but he is nearly 8 and slowing down a bit. I would agree that 60kg is a fair expectation for a Rottie ( My lad has the GSD legs, face, a little broader, and ears so the legs are a bit fragile for the rottie body) I also thought your pup looked a bit thin but some pups are hyper and burn their food off before it can turn to "padding" 20kg is still very underweight for his age, I would expect him to be 30 to 40kg by now and 50 to 60kg by 18 months old. A rottie's fur is the most shiny fur I have ever seen on a dog. They are so glossy and gorgeous, a dull coat is always a sign that there is something wrong, either better food required or maybe illness. When I changed my boys feed their coats went quite dull. People still say they are glossy but nothing like they were on the previous food. Unfortunatley £48 every 3 weeks is a bit steep when £12 is the alternative.
Have a google on homecooked food for your dog. Not just left overs but actual meals especially cooked and designed with your dogs nutritional requirements taken into account. I have a lot of info on this as this is all my boys ate for years and it was not that expensive PM me if you want some links or recipes
Get him checked out, there must be a reason for him being so underweight. The worry is that he might have health issues later on in life due to malnutrition in his formative months. Ask if a calcium supplement or multi vitamins would help. Ask for a second opinion!!!! I do hope that he will be fine, I come across so many badly starved and neglected dogs in rescues and pups can have issues for the rest of their lives if they did not get adequate or proper nutrition whilst growing.