I think around 18 months toddlers need a little less sleep. It's just not as obvious at this age as when they were younger, because instead of dropping a nap you have to tweak the nap/night time sleep schedule. My son is 20 months and now needs about 30-60 minutes less sleep than he did at 15 months.
2 ways to do it are
-shorten the nap by keeping your son up a bit longer. Watch him for tired signs rather than by the clock. It's tricky to catch them when they're tired but not overtired! Try to get him up by 3.30-4 pm. Keep the same bedtime.
-keep the same naptimes but put him to bed 30 minutes later.
It sounds like things have gone off track though, so maybe the first option of keeping the same bedtime and a later nap time might work. It might take a few days though. good luck!
I think the amount of sleep needed shortens again at around 2, so I suppose I'll be having to do this again soon!