That one ear up, one ear down is SO cute with the rest of his looks.
I would guess he's a mix, as well, but I think it's something like 3/4 GSD, 1/4 something else, or even a higher percentage of GSD because if it weren't for those fuzzy ears I'd swear he was pure GSD. Those big GSD feet look like he's got a lot of growing to do yet.
He certainly weighs less than my six month old GSD (see Damaged Ear thread that I started in late March and just updated) but my pup was in the vet's office last month with another GSD pup who was a month older than mine, and Jack was ten pounds heavier. So there can certainly be a wide variation. So I don't think I'd be concerned with what he weighs now. If he is a mix, probably whatever else is in him was a smaller breed, is all. And like I said about his feet, he's not done yet by a long shot!
As for feed and picky eating, mine went through that as well. Now, he was younger, but there were a few times I was sure he was sick and then the next day he was eating again normally. If he has a food that he really likes and devours it, you can probably figure he's just being picky. But you are smart to be thinking about a visit to the vet just in case.
Good luck with him, gosh he's so cute.