He’s seems a bit young for thyroid problems, it’s usually around 4-10 yrs. But, anything is possible. It wouldn’t hurt to run a test (T4) this way you would know for sure. If it is a thyroid issue the vet can prescribe an inexpensive medication for your dog. Losing hair could also be Alopecia or pattern baldness.
If it’s not a thyroid problem you might try changing to a top quality food and adding a little Salmon oil to the food and see if that makes an improvement. This would take at least a month to show a difference if any in his coat. Although if it is Alopecia or pattern baldness there is nothing you can do to correct it. This is one of the problems with the Dachshund breed.