Mystery Lamness May kill this horse!! help

to start we have had pros form all over hear this and ahve no clue. but you know it can tak another look to see something we missed... you never know... so here goes
Kudos is an 18 year old TB Hunter Jumper school master

. he has been lame for about 5 weeks now. he first presented dead lame on his right hind would not move or eat or stand for 2 days without encoragement and pain killers did nothing for him. we took x-rays of his sesinoid, some of his foot, and his stifle for fear of a fracture .vet was out hoof tested him with no reaction, flexed him with no reaction. x rays came up clean except a dark pocket near the outside of his heel. Vet said it was an abcess and to treat it as such. 3 weeks later farrier comes out to do feet and trims, hoof tests him again. still nothing. he did in the meantime blow out 2 abcesses in the front heels that he was not lame with.
we have soaked, amd polticed it ALOT.
for the first 4 weeks streight with no luck except the heel cracking and the hoof wall seperating in one spot jsut a little bit. but no drainage or reaction site.
today he will get around the pastures and limp around even attempting to trot and canter, not well mind you. he does not have a droped hip, but does not seem to want to flex his stifle or bare weight on his heel.
he has now lost a ton of weight and muscle.
I am getting afraid i will have to put him down. but when i call the vet he jsut asures me as soon as teh abcess pops he will go back to being fine again. I really dont feel this si fine. any info would be great!
I am wondering if it could be afemoral nerve issue. or worse a fractured pelvice (though he flexed fine)