Welcome to Yorkie Talk and congratulations on your new baby. His size is normal, but on the small side and requires extra attention.
I agree that he needs to see the vet again to rule out mites, ear infection, or reaction to flea bites. With your pup being tiny, anemia is a potential risk. Fleas also carry tapeworms and can cause secondary skin infections that require antibiotics.
Dog Flea Treatment | petMD
In addition to the vet, vacuum your home thoroughly every day, and wash his bedding as often as possible. Fleas lay eggs quickly.
Although oatmeal shampoo can be soothing to some dogs, it can also be an irritant to others. Try a different shampoo. Lather up your pup well and let it sit for a minute or two, massaging your pup at the same time. This will help kill fleas and remove flea eggs. Follow up with a conditioner to soothe.
At his age and size, I don't know which if any flea preventative would be recommended. Talk to your vet.