I think 30 kgs is a decent weight for a GSD, even the topmost quality doesnt weigh over 35-40 at 3 yrs of age...it also depends a lot on the genetics of the dog..still you can try the following measures:
1, Give ample amount of exercise (Not running, i am talking abt him dragging you when u take him out for walk, this will improve his chest and hind legs to the most.)
2. Make sure that deworming is done on time .
3. I would suggest you also get his checkup done for his liver functionality ( he may have some liver issues ) and giving Liv52 will solve this issue (pls consult your vet first and then only go for it)
4. Make sure that he gets enough water all through the day.
5. He is mature now, get his mating done...most of the dogs gain weight/muscles after 2-3 matings at regular intervals ( NOT BACK TO BACK).
Hope this helps.
Thanks & Regards