It may be a temporary "nursing strike" - kids will rarely actually self-wean before about 18 months. Sometimes kids do this if they have a cold or ear infection, or just because they're feeling stubborn.
I agree cutting out the afternoon snack should help. Is your nanny giving him juice at all? You can cut this out too, as it really has no benefit and just fills him up. As long as he's getting a couple meals (of breastmilk) in each day he will still be getting benefits from it. I would continue to offer him the option to nurse frequently and chances are he will pick it back up again.
Are you still pumping at work? If you have a pump at home I would probably go ahead and pump if he skips a nursing session - that way he can have the milk later in a bottle, cup, or mixed with cereal. Plus, it will keep your supply up until he decides to nurse again.