Hope he's OK
but watch him carefully if the sting is inside the mouth as occasionally stings in the mouth can cause a severe reaction with swelling that can inhibit breathing severely, plus some dogs (not many I believe) can suffer the same sort of extreme reactions that some humans do and go into anaphylactic shock.
What's much more likely is a bit of swelling and some discomfort for a few hours. Giving a child size dose of Piriton can help, but note that for dogs it has to be Piriton,
not the non-drowsy products such as Piriteze.
If the sting isn't in his mouth you could also treat with Bicarbonate of Soda if it's a bee sting or Vinegar if it's a wasp sting.