my horse is sick

Question
i am 13 years old, and ive been riding horse since i was 6.
my horse is sick and we don't know why. it started the other week when he sounded like he had a cold but then he had trouble backing up so we called the vet out and he said the horse might have string-halt, he also said to lock him up because string-halt can be caused by what the horse might have eaten in his paddock. so we locked him up and then the vet said to ride him but only walk him because he would need excersise from being locked up all day. we went riding  and about half an hour into our ride while we were walking he just callapsed and had like a fit. he was really uncoodiated, he was groning and he was twisting and turning. he went from one side of the road to the otherthen he went over backwords and wacked his head and then he went unconcious. he rolled his eyes over and he loooked lke he was dead. we run back to the house when we here him neigh, we turn around and he was troting towords us. he has gashes and cuts eveywhere. he has a swollen eye and leg, the vet couln't tell what was wrong with him and we don't know if its a virus or he will start geting fits because his only young- seven. should we get him put down or what?    
we normally leave him in a 1 acre paddock.

Answer
Hi Miranda, sorry to hear about your horse.  I am not a vet, but when a horse does what you are explaining it sounds serious.  I would not put a horse down unless he was in pain and had no quality of life.  Too many things that this could be so I can't begin to guess.  It could be a poison plant he ate, could brain tumor, if a horse eats bird droppings or eats grass with possum urine on it, it can cause some the symptoms you are explaining.

I would put horse in paster with lots of fresh clean water and free choice hay.  Put hay out so he can eat when he wants, as much as he wants.  Make sure he has a salt and mineral block so he can use it if he wants.  Keep an eye on his temp so you can tell if he runs a fever.  Give him short walks if you want just to spend time with him and monitor his symptoms.  

Horses get sick and as long as they drink and eat they can bounce back, if he stops drinking he will dehydrate and other problem will occur, so keep an eye on him and keep your vet informed.  I would not ride him for at least two or three weeks.

Hope this helps,

Rick