For us, waking crying has always meant that ds needs more sleep.
Your ds might be waking due to being too tired when he goes down for his nap and hence has a broken sleep cycle or that he's gotten used to wake at teh end of the sleep cycle and is not able to go back to sleep.
I have 2 suggestions here:
1. Try and offer the nap 30 mins early and see if he'll sleep longer or at least wakes happier. Overtiredness is the main reason why lo's wake in the middle of sleep cycles.
2. Try wake to sleep at the 1hr 20 min mark of sleep. It is described on the FAQ section of this board. Try at least for 3 days in a row and see if he can sleep through on day 4, if not, try wts for another 3 days in a row.
You can try this suggestions separately or together, as you consider best.
HTH and keep us posted!