My Horse is sick and I dont know whats wrong

Question
I noticed when i was riding my horse the other day that he was acting a little funny, like something was wrong because he was putting up more of a fight than usual. He is 14 years old, and last night he collapsed and never could get back up. I noticed his eyes are swollen and really red. I could see when i went to check on him that he had tried getting up through the night because of all the marks on the ground around him. I don't know what all i can do the vet came to look but they cant tell what is wrong with him. I don't want him to suffer, so should i put him down?

Answer
Stephanie,
  I am so sorry to hear about your boy. What exactly did the vet do when he came out to examine him? Did he draw blood or do any other tests?
  I am not a vet, but my feeling is that if your horse can't stand it could be serious. I would call a vet whose opinion you respect and discuss your options at this point.
  You and your horse will be in my thoughts and prayers. Please let me know how you make out.
  God bless!
Anne Stepien