Hey Helen
I too was thinking wow, that is alot of sleep. Maybe his night needs to be a little shorter? It is 12 hours right now, but I think many this age are doing more like 11-11.5 - so instead of shortening the nap, maybe allow him up a little later, or wake him earlier in the AM, move the nap earlier, then bedtime same time?
Just a half hour change might make the difference.
Also, when Zara was doing that to us (she did it at 12mo) we decided things coudn't get any worse, so did the big bed switch and sleep trained in that.
She too got too worked up with WI/WO, and played if we stayed.
So we modified and did a combo.
We put her down, gave a kiss, said good night etc, then slowly backed out of the room, shhing her if she moved, she would keep looking for us. If she cried, we returned to her bed, kiss, night night etc start again. If we made it out the door, it would take us several minutes to shut the door...and if she cried, we returned immediately, no counting. If during this, she tried to play or engage with us - we would say No Zara it is bedtime if you are going to play we are leaving...and would leave the room and close the door - she would instantly scream and we would immediately return, no counting.
But she got the idea that if she plays, we dont' stay, but we will stay and help her if she tries to go to sleep.
Does that make sense? Let me know if you need any further clarification.