It sounds like your furbaby has the exact same thing going on that my Gage does!!! He has lots of hair around his face, chest, legs, belly, and butt but slim to none on his back. I can tell you that I've had his thyroid checked (normal) and had him spayed (vet said sometimes hair loss could be testicular related and having him spayed may cure the hair loss) and he still has no hair! Otherwise, he is a very healthy and happy little yorkie
I have clothes for him to wear when it's cold to help hold in his body heat and he has t-shirts to wear when he goes outside in the hot sun (all advice of my vet but I already had the outfits cause I think he looks so cute in them!!). Vet said to either put a shirt on him when he goes out or sunscreen - a t-shirt is less greasy. My vet said there are more tests he can run to try to figure out what could be causing the lack of hair but I have personally chosen not to put him through all that (skin scrappings, blood tests, etc.) - not only from a financial standpoint but if I don't have to put him through it, I choose not to. I fell in love with him when I rescued him from a bad home and he didn't have hair then so it doesn't bother me. I've never posted pics of him without having some type of clothing on but I will try to take some tonight of him and share tomorrow so you (and others) can see what I'm talking about.
If you've had your furbaby checked by the vet and he's healthy otherwise, then it becomes a personal choice on whether to have tests run to find out why, in my opinion. Best of luck to you and can't wait for you to post pics
Oh, and welcome to YT!!