The redness and increased warmth is due to an allergic reaction
of the wasp sting. Keep a close watch on his condition. If his breathing is getting hampered he will need immediate medical attention. He can have Benadryl
or Phenargan to tide over allergic symptoms. It would be best to apply a mild antibiotic ointment like Neosporin
ointment at the site. If it does not improve, kindly get him to the Doctor as he may need intravenous antibiotics.
Wishing speedy recovery.