My son (8 months old) has few empty patches on head with no hair growth. Is this normal? What is the treatment for this, please

Normal. It's normal to have some spots with no hair, especially the back of head because it rubs when the babies lie flat. If the patches are accompanying dryness like white flakes, it could be seborrhea. You just need to give him good massage with baby oil and takes out the dry flakes.
See below. Bald patches on infants are very normal, especially on the back of the head. There is no treatment required.