It all depended on the situation for us. If we were invited to a friends house where I knew there was somewhere for the kids to sleep, then we had no problems with going, but then none of our friends are the loud music/loud parties type. Lindsay also had the same routine as your DS and we found he could sleep anywhere anyway. We were at out local pub once (mind you a busy night is 8 people staying until 11pm LOL) and someone decided to put the jukebox on and he slept right though, although we did leave soon after that anyway.

I totally understand why you don't want to do this to your son, so just pick and choose your 'parties' as such and only go when you know the environment will be better IYKWIM? If your friend wants you to go out, can you arrange a babysitter for the night, that way you can go out but DS still gets to keep his routine?