I have a mini black/ tan dachshund and he is 9 months old and seems to me to be too skinny. What is the ideal weight for one?

That varies. Nobody can give you the right weight for your dog. You can learn how to determine what he should weigh at LongLiveYourDog.com - Life Span Study - Rate Your Dog Remember, lean is healthy. Don't ask anymore of his back than you have to.