help.... poorly pony

[celtes]

Hey. My friends pony isn't well and we're having problems finding out what's wrong. His face has been swelling up and he has a bad cough. He seems to get better when he's inside but worse again when he's put in the field. I was thinking maybe it was St John's Wort poisoning as that causes light sensitivity, but all the sources I have seen say it affects white patches... he doesn't have any!!
The vet has been called but he was most unhelpful saying that they don't do antihistamines for horses and that it was probably an allergy.

Help!!