I think the odd day napping in the car or buggy is fine especially as he sounds such a good sleeper
I tend to stay in for naps when I can but that is my choice and mainly because I like the break (!) and also DD is very spirited with sleep and does not sleep well out and about, never has done.
Having said that this summer we have some days planned and she will nap in the car, we just do an early bedtime if she short naps - days out are important