I would try the bottle with KMR and some Nutri-cal from the vet. Maybe he just wants to "nurse" some more.
Did you physically tell the vet how old you were told he is? Did the vet coment on his age? Sounds like he could be younger and if you didn't mention his age, maybe the vet assumed you knew he was younger and thought the weight was okay. I am just trying to think of all possibilities here.
I find it odd that the vet didn't say anything about the fact that he weighs 1.1 pound at 10.5 weeks. That is crazy, maybe find a new vet if he thought that was okay.
Did you physically tell the vet how old you were told he is? Did the vet coment on his age? Sounds like he could be younger and if you didn't mention his age, maybe the vet assumed you knew he was younger and thought the weight was okay. I am just trying to think of all possibilities here.
I find it odd that the vet didn't say anything about the fact that he weighs 1.1 pound at 10.5 weeks. That is crazy, maybe find a new vet if he thought that was okay.