Sorry its taken me so long to post again, started a new job this week and haven't had ANY time to be online!
We did see the rest of his litter, he was only the 2nd one sold out of 8 pups, and he was middle sized. The biggest one was twice his size, and the runt was half! (she was adorable, we almost picked her) The rest ranged in between, mostly around his size. He was one of two brindles, the rest were fawn.
The BYB we got him from told us he was having trouble selling them because there were about ten other litters listed born around the same time as him, so there was an abundance of boxer pups in that area at that time. There
were a bunch in the newspaper that weekend, we looked. That said, seeing the pictures you all have posted, I have to say he really does look more like 8 weeks than 12. Both parents who were on site looked about 60-ish lbs, the female maybe was 55 lbs. So I don't know that he will be tiny, if he follows in their size.
Which brings me to my next point. He came with papers, and on his registration papers it states march 19 is when the litter was whelped, which means he would have been 12 weeks when we got him June 17. But the papers (on closer inspection) also say he was in a littler of NINE puppies, not eight. There were only eight when we went and got him. (the one that had been sold had not been picked up yet so we got to see that one too). So I am thinking this was a little shady. Sigh. Not that I regret him, I love him to death but I wish I had known better, I hate that we gave money to a BYB this irresponsible. And the phone number that we had for them is now out of service, so we have no way to contact them.
The vet has never told us what he thinks, he just asked how old he was and made general comments on how adorable he is etc, although they did think he was a Pit when they first met him. He didn't have very droopy jowls when he was smaller, they have finally started to drop and look more "boxer." Of course the vet also said that he had never seen a boxer that color and that he didn't know they even came in that color

so obviously he was not a boxer expert! But he was very kind and willing to listen to us and our opinions.
Anyway, sorry this was so long. To summarize, do you all think we got duped? I am getting the feeling we did....