We had a big language leap at around 21 months which threw sleep off for a while. If it's a recent thing I'd probably put it down to that and ride it out, maybe even put down for bed a bit later to reduce the amount of time he is delaying sleep and once the language leap is over everything should go back to normal.
If it's going on a longer time I would think about capping the longer naps. It may appear his BT antics do not change with the varying length of naps but his amount of sleep across a week would effect his overall level of tiredness and he would be more likely to go to bed on time - rather than looking at individual days/naps and BT on the same day. Hope that makes sense.
Those are my thoughts.