M.
C.,
One of my children gave up naps entirely and only slept for eight hours a night--by his first birthday!!
I would definitely do the quiet-rest time like the other mother's suggested. If he naps, he naps. If he doesn't, he doesn't. Even if he does not go back to napping, it sounds like he still needs rest time.
You could also get him up a little earlier or keep him up a little later (shorten his nightime sleep) and he may start napping again.
M.