We are in the same boat. If I let DS sleep any longer than 30 minutes in the am, we get pm nap refusal. I'm trying to adjust though to waking him after 30 minutes. It is so hard that I've been letting him sleep 1-1.5 hours, and then of course getting problems in the afternoon.
If I wake him after 30 minutes, he can easily go 3.5 hours before having a 2 hour nap!