Pay it forward said it best.
try not to get too upset at the unknown, but it is good to have a plan.
Most importantly, do not let him roam if he is positive. We (humans) must try to contol this disease, as he could infect the next cat that comes looking for a safe home.
Try all shelters and rescue groups to see if they have or know of someone who has a leukemia positive colony. That is what friends of mine did when they happened across a positive kitty.
And if you do decide to PTS, try to remember that you do it with love.

BEST of luck at the vet's