he was told open-toed shoes/sandals were regarded as offensive. Is that true?
Only if his feet smell...
Seriously though, just tell him to behave as he would in any other major city and I'm sure he'll be fine. Stay out of the bad areas, don't look like a victim, stay alert, don't stare at strangers, blend in... etc, etc, you know the rest.
I'm no fan of big cities, but Londoners often get a bad rap for being unfriendly. I would say that I've found them to be reserved, yes, but generally not unfriendly. But then, it takes all sorts to make a world and you can probably find most of them in London.
As for the weather, we hit a record 100F a few summers ago, but the weather's pretty unpredictable, so your son should pack accordingly.
Hope he has fun, anyway!