I think 15 months is 'late' as in everything is aimed at them walking at 9 months, but I don't think it is worrying late IYKWIM? I think if he actually wanted to, and put his mind to it, he could walk, I think a lot of it is lazyness, he crawls at the speed of light, so he thinks if he can get whereveer he want's to go by crawling, he doesn't need to walk does he?! If he hadn't been standing/cruising by now, I'd be a lot more worried, but he's been doing that since 8 months, so I haven't got a problem with it yet!
Your poor DS though, it's not on is it? As you say they should know him better by now. I still worry about DS1 being at school! I remember in the nursery one of the teachers couldn't ever even remember his name, kept calling him by our surname (which is also a boys name) but you'd have thought it wouldn't be that hard after a year of looking after him to remember his name! And his name is Thomas, it's not some fancy-pant's made up name that no-one has ever heard of before!
Anyway, I hope your DS is feeling better today? Could he have the same bug as you?