I would say that Zyrtec is not a cure for scabies,the only recommended medication of choice is 5% Permethrin cream
Zyrtec is an anti histamine
and and can only help in reducing the itch caused by the infestations.
If there is no one else affected in the family it doesn't mean that Scabies is ruled out,someone in his school or may be some other kid in your neighbourhood may be affected.
But if you are sure that there is no possibility of Scabies then you should see your dermatologist
to review him for Urticaria
or Contact or Atopic dermatitis
I hope I have made it clearer to you.
Thanks and best of luck.