Hi again and thanks for the info.
It is possible that your guy may have a very fast metabolic rate causing trouble gaining weight. This is common in german sheperds and this may be normal for him. It would be unusual for such a young dog to have a thyroid problem causing this inability to gain weight but it does occur. If nothing works to put on weight, you should at some time have him tested.
It may also be a prob elm of scarring in his intestinal tract due to a heavy parasite load that is now gone. This will cause your guy to not be able to draw in the nutrients as well as other dogs so he needs more food and nutrients.
The best thing to do for now is to increase his food intake using a high quality food such as Science diet
for large dogs or Royal Canin. Add into his diet high calories canned food
also add add in 1/4 cup vegetable oil to his food every day. This will add pure calories to his diet. I would also suggest you purchase from a pet store a product called Nutrical and give your guy 2 tablespoons of this per day. This is a high calorie supplement that also will add calories.
If, after 1 month, he does not put on weight, you will have to have your vet examine him. I do believe, however, that he has a higher than normal metabolic rate and this is just him.