My Horse is sick?? What is wrong with him please help?

I have a horse who is 5 year old thoroughbred he has been sick for seven weeks now at the start he just had swallon legs only got all the rest of these symptoms about two weeks ago we have had heaps of vets come and see him to find out whats wrong with him....And no body noes whats wrong with him its getting worse and worse every day now their thinking about putting him down....Which I dont want to do cause i already lost my mare earlier this year to a snake bite i am only 15 and ive been having a hard time dealing with it i dont no what will do without him either... Okay so here are his symptoms please help i really need all the help i can get his name is ziggy..He has excessive swelling in all four of his legs as they are solid all the way up.. little cuts that never heal and. he is losing his hair everywhere.....panting to breathe...snotty nose....bloodshot eyes. cant move....when he does move its just to get water but hobbles along....extremly skinny. He still wants to fight it i can see it in his eyes... if anyone noes what is causing this it would be muchly aprreciated please no answers just saying put him down i need geniune advice from a horse owner or vet or just someone that noes about horses in general please help