I couldn't sleep without telling you of this technique that has worked with Nick....Its based on ABA therapy & it would go something like this..
#1 - Hopefully your son has a favorite "treat" - take it away for a couple of days (stall, substitute, whatever you have to do)
#2 - when you're ready to begin, reinforce the "positive" (i.e. keeping clothes on) by "catching" him doing it right (every 10 minutes, every 20 etc.). Reward him by giving him a treat (i.e. 1/4 candy bar, couple of skittles - whatever the reward - just not the "normal" amount). This may mean he gets nothing to eat but the treat for the first day or 2 - but don't worry - you will gradually reduce the rewards (from every 10 minutes to every hour, etc. etc.).
#3 - at the same time - if he does get naked IMMEDIATELY remove whatever he is doing (i.e. if he's watching t.v. - turn it off & say "sorry - naked boys can't watch t.v.; if he's playing with his toy - remove the toy - doesn't matter if he plays with another one - if he gets naked again - remove that toy too until there are NO toys ). Once he's put back on his clothes - immediately return the toy he was most interested in playing with with praise (that's great nick - your dressed!).
#4 - with this technique - there is absolutely no "punitive" lecturing or punishment. All is to be done as if you couldn't care less.......and any temper tantrums that ensue are TOTALLY ignore (he's not even in the room - you can't hear a word he's saying or scream - your reading or whatever). This sounds "stupid" - but in reality it REALLY WORKS. Eventually they learn that temper tantrums do not get them anything and they stop throwing the temper tantrums since it doesn't do them any good.
Do not worry that he will become expectent of a reward - it doesn't happen as you gradually reduce the "reward" - eventually it just becomes part of his normal life and He won't become reliant on rewards.
It takes ALOT of consistency here, but the therapy is sound for all individuals - not just those with autism. I find I use it with my other son as well - ignoring bad behavior while removing the reward for said behavior, but constantly finding the "good" stuff & strongly praising it. Sounds funny I know - but I can tell you it works. We have settled many behavioral worries with this technique (nakedness, smearing feces, temper tantrums etc. are just a few).