fact of the matter is, anyone will fall asleep if someone rocks them long enough.
my son is 4 and he still needs an occassional nap. im wondering which way you mean he doesnt take naps; does he not go to sleep (as in fighting it) or does he go through the entire day with energy, good attitude, etc? my son is a bear some days, and gets such a bad attitude that he needs to sleep, and usually those days if i can get him to stay in his bed and just chill out, he does go to sleep.
a lot of kids need much more sleep than they are getting, but it sounds like hes getting a good night's sleep on a regular basis. during times of illness he might need a nap, but you probably know and understand that.
what i would do is this : the next time it comes up, simply say "ive spoken to my doctor about this, and between the doctor and us (is dad involved?) we have made the decision that his sleep habits are just fine, he doesnt need the additional nap, and it hurts his cycle if we have a nap so close to bedtime. the doctor advised us against nap so late in the evening. if he needs a nap, ill be sure he gets one after lunch."
and then just dont be open for the discussion. you COULD ask them about their experiences with kids and naps; if they allowed their own children to nap in the late evening, and then what time they went to bed after that.. etc. do it in a way that is polite and interested (even if you are not interested). it will at least make them feel as if their experience is valued by you, and might help them to back off a bit. if you dont want to ask about sleep, ask about something else you might be interested in hearing about. i think they might just want you to appreciate their experience in some way. i know it can be a double edged sword because once you ask for their opinion you might actually open the floodgates to their whole entire opinion. buuut, just be polite and matter of fact no matter what you choose.
one thing is for sure, if your son has regular bedtimes and regular wake times, hes got a very healthy sleep habit going on! after all, they tell adults all the time that irregular sleep habits can cause insomnia and trouble with sleep, and bad health. so you've got it going on.
but anyway, i know you are frustrated and hurt, maybe even insulted.