Two hours morning and afternoon
How I know he's droppping a nap: he now won't go to sleep for his morning nap till 10am (wakes at 6.30am). He hasn't routinely slept for more than 35 mins for this nap for 2-3 months. (If he does sleep longer in the morning, he'll only sleep 45 mins at the next nap, instead of 2 hours.)
I think the first nap getting shorter is often an indication that they're starting to drop a nap.
He was having the second nap 1-3pm. Now he can't go to sleep before 3pm. Then he still wants to sleep for 2 hours, but that means he can't go to sleep at bedtime before about 9.30pm or later, which is definitely too late.
So the compromise solution is to keep the first nap at 10am, and only let him sleep 45 mins at 3pm, so that he can be asleep between 7 and 7.30pm.
It's not enough sleep for him overall, but I'm taking Elizabeth Pantley's advice of sticking to two naps for as long as possible. On the positive side he seems to be tired rather than overtired - we've not had early morning wakings and only one night of being up for an hour and a half. He seems to be coping (so far!) better than with the drop from 3 to 2, when we had both those problems.
The length of awake time before he gets sleepy again seems to have lengthened all of a sudden rather than gradually. It took me by surprise a bit!
IIRC the Weissbluth stats, 4% of babies are on one nap at 12 months, 50% at 18 months and 96% at 24 months. So your DS is well within the normal range IMO.