^^As mentioned above, when you are sure he doesn't have some physical problem that is preventing him from picking up that lead (farrier, chiropractor, dentist [teeth CAN affect this!], etc) than you can start "pushing" him to get it.
Make sure you can first side pass and two track him with your legs, so that he knows how to respond to your legs.
You want to keep his nose tipped to the inside when you ask for the right lead so that his body is on the proper bend. You may cue him for the right lead 178 times before he actually does it, but keep on trying!
And as someone else already menetioned too, when he does get it, it probably will be pretty rough, as he hasn't worked those "right lead muscles" in a very long time!
Since he does pick up the right lead while lunging, you can supplement that as some exercise to help him build those muscles on that side.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.