You do not need to feed him 6 times a day. Many GSD pups have high food drive - meaning they will eat as much as you give them. Go by how the pup looks rather than if he behaves hungry.
Stop mixing his food with milk. Dogs cannot metabolize cow's milk and pups do not need milk at 7 weeks.They are weaned by then to solid food and can exist just fine on solid food only. If you insist on giving him milk, give him goat's milk - but again, it is not necessary for a pup of this age.
Give him 1/2 cup to 1 cup of food three times a day. After 8 weeks, give him 1cup of food twice a day. Mix it with water, let it soak for 15minutes and feed him. If you want to add any extras, you can add a raw egg for one meal, a spoon or two of salmon oil, or add some ground meat with no bone to one meal.
Large breed puppies need slow sustained growth for optimal health. Do not overfeed your puppy. There is no "ideal" weight for a puppy because each puppy is different.
Make sure your puppy is wormed and free of parasites. Parasitic infections and worms can cause excess hunger or thirst. Good luck to you.