Thank you for your query
A
rash can occur due to a number of reason , a couple of which you have excluded for me.
Am
allergic reaction is one of the causes for a rash. Others may be
Chicken pox,
folliculitis,
warts,
scarlet fever , to name a few.
The appearance of the "pimples", the type of progression (spread to other parts or locally), if any fluid in them, if it is is associated with fever or pain and circumstances of his exposure in the past week or two would help with pinpointing the cause.
I recommend that you see his
pediatrician who will be able to judge the rash based on the above and administer medications ( if required)
In the meantime, keep the area clean and dry. Try to notice if anything makes it worse or better. If it itches or has any discharge.check his clothes and the area where he sleeps, sits and plays for any bugs.
If at any point the rash worsens and he has a temperature of more than 102F even after medication to reduce fever, take him to the ER right away.
Hope this helps.
Please let us know if you have further questions.
Wish you both good health