To me 65lb for a 15mo is pretty lean, he does have a nice trim waist which is what you want. 65lb is a good average weight for an 8 to 9 month old.
Not sure if 350g is enough food for him. Do you feed anything else as well as the dry kibble?
If you feel he is getting adequate amount of food each day, then keep an eye on him and if he does not gain weight over the next month then I would have him tested for EPI EPI in Snapshots - EPI * Exocrine Pancreatic Insufficiency
and also SIBO.
I have a 3yo GSD male that was diagnosed with EPI last year, at 2 1/2 years old he weighed and looked like a 1 year old. I thought he was a good trim weight but soon realised something was not right when I realised he weighed 32kg at 2 1/2yo. So I had him tested for EPI and he is now on replacement enzymes and weighs a healthy 39kg (85lb).
Not trying to scare you, just something to consider.