Those little needle teeth will be hanging around for a little longer yet. He'll be at least 16 wks before you notice his incisors falling out and being replaced by his adult teeth. He'll be almost 6 months old before his adult canines come in. All of his adult teeth should be in by seven months old. Watch though with his baby canine teeth - dachshunds have a bad habit of retaining those teeth and they'll need to be removed when they go in for spay/neuter because they can cause problems in the future if left in.
Now for the thinning hair - he could be starting to blow his puppy coat, meaning that his adult hair is coming in...but he's a bit early for that. Hair is thin to begin with on the ears - especially in smooths. You could try adding a little fish oil or getting a puppy food that contains salmon oil in it. It should help.
