That sounds like ALOT of kibble. Especially since the kibble is a fairly high calorie quality food.
The afternoon cowpie could be because it's too much food. When we over feed our dogs too much they can't absorb it all and they produce pudding poop. Exercise can also cause poop to become loose if he has a big run before he produces that poop. If poop is consistently loose and he continues to loose weight there is the possibility of parasites, or he's not properly absorbing his food- so you could look into EPI.
Does he just feel a little thin, or is actually clinically too thin? The reason I ask is that MANY GSDs at that age go through a thin and lanky stage that is appropriate for their development. Tag went through a period where we called him Skeletor because he felt so thin under his coat. This is Tag at 10 months...You can see that he's pretty thin and not at all filled out.
Cade (on left) was skinny at that age too.
Anka was also pretty thin. AND she didn't want to eat. If I tried to feed her more, she just wouldn't eat it. Sort of to say "No thank you, I've had enough"
Argos was not. But that was because I fed him like he was a Christmas goose. He was actually a little fat