If you can't find anything and you don't want to send him to a slaughter house, it'll probably be in the best interest of the horse to put him down. Unless you can afford pain medication for him.
A lady I know was lucky enough to find a therapudic stable in the city who was willing to take her gelding on (he had severe navicular at age 10).
My other friend's parents also have a gelding with navicular. He hasnt been ridden in about 3 years now but I know he'll stay with them until he dies, which may be awhile seeing as how he's only 14 or so & in good health otherwise.
They're lucky enough to have plenty of land, though & her dad makes their own hay & does his horses feet so expenses for them are very little.