Yes you're absolutely right. As they get old, they need more awake time in order to be tired enough to go down easily. 3 hours awake time is enough for an 8 month old but I beleive that a 20 month old needs at least 5 hours. Besides, he can be acumulating overtiredness, since he goes down for his nap so late. What time does he wake up? By all means, try to move his nap backwards, depending on his morning wake up time, make it at last 6 hours after he woke. Then provide at least 5 hours after waking for nap, so he's tired enough to go down and stay asleep.
On an extra insight, I'd recommend you to stop counting to a certain number when he cries out for you. It is better to listen to his cries (is it a whine? is it a scream? is it an hystericall crying?) Go inside his room as soon as the cry starts to sound unconsolable and the type of cry that you know he's gonna need your help. That will show him that you'll be there for him whenever he needs you and will prevent any separation anxiety issues.