It could be allergies. Yorkies are often allergic to corn and/or wheat in their food. Sometimes it is the shampoo you're using.
Mitzi had that too and after changing food and shampoo she's doing just fine.
It might help to put a cotton shirt on him to prevent him to scratch and cause inflamation and irritation.
Hope your baby is getting better really soon!
Ohhh... and WELCOME TO YT!!!!!