Thank you for your query.
A spot on the scalp
from where hair have fallen and which is red and painful to touch should be shown to a doctor. It appears to be an infection and needs to be treated. It could be the start of a fungal infection
and is amenable to treatment. Shampoos are not going to be effective on them.
I hope I have responded to your query to your satisfaction.
Wishing your son all the best.