You do not mention what kind of dog you have but even if the dog is a large breed 10 pounds is significant. There are several reasons the vet told you to just "wait and see"
1. He already knows what is going on and didn´t want to tell you
2. He figured you would not want to spend extra money on the diagnostics
I would recommend you call up the clinic and tell them you are not satisfied and need to know an answer to what is going on. If they ask for a recheck and then do bloodwork it will be less expensive than another vet, if they still come back with the "wait and see" answer I would get a second opinion. There are a lot of medical conditions that can be treated early. Treatment might significantly increase your dog´s lifespan.
Do not accept "just old age"!