I have a 10 month old, neutered male GSD that is just not gaining weight, and as this is my first GSD I'm not sure whether I should be worried.
He is approximately 26 in. at the whithers (give or take a few in. for wiggling around) and he has been weighing just under 60 lbs for about the last month (59.5 last time I weighed him). He does look thin (tucked waste and abs) but you can't see any ribs except occasionally the last one. I've tried feeding him more but when I do he just leaves the extra in his dish.
Other than the weight problem and a little pano in his left fetlock (are they called fetlocks in dogs?) he seems to be pretty healthy. I've wormed him recently and he's got bright eyes, lots of energy, and a thick shiny coat.
NEITHER DINKY OR ME HAVE ABS LIKE YOUR DOG BUT TO GAIN WEIGHT MY GF STINKY USED TO GO TO THE DAY OLD BREAD STORE AND GET ALL KINDS OF LOAVES OF BREAD. FOLKS MAY SAY ITS NOT GOOD FOR A DOG BUT DONT LISTEN. IT MAKES DINKY FAT AND HIS COAT SHINY. IT GIVES ME BIG LOVE HANDLES FOR THE GIRLS TO GRAB HOLD OF AND HANG ON. PUT SOME BUTTER ON EACH PIECE AND THE WEIGHT WILL TAKE OFF.
Running your hands down his sides you should just be able to feel the ribs, not a layer of fat. If you can barely, sometimes, see a rib he is probably not too thin. Too many dogs are too fat so that's what people expect to see.
Try changing to a different feed. The ingredients seem to be changing without notice and dogs know what's good for them or not.
I used Eagle Pack for about a year dogs loved it, bought a new bag and the were off their feed. Wouldn't touch the stuff. Looked out in the yard and it was full of puddles of poop. Don't know why, but I haven't purchased another bag of that stuff.
Something with more carbs, or try topdressing with a few table spoons of "drippins" from what every you happen to be eating.
"He does look thin (tucked waste and abs) but you can't see any ribs except occasionally the last one. " I don't see anything wrong with this description.