No one in our group of friends/parents have sleepovers at that age unless the families are particularly close. They tend to start at around 9 or 10 for friends houses, although staying at GP etc starts much earlier.
It might be 'totally normal' for some, but definitely not for others!
You've said that you don't know the family well at all. I slightly amazed that so many people would happily pack their 7 year old off to someone's house who they barely know!
I'm iroically amused that you (a) dont know the parents very well (b) let him go round for dinner unaccompanied. What will happen at a sleep over that wont happen at the dinner table? Do people morph after dark or something?
How very MN, with the barely veiled 'oh you think your child will be harmed, don't you?' sneer.
In rl, there really is a very big difference for a 7 year old between having a short play and a bit of tea with a friend and staying at their house for the weekend, with unfamiliar routines, sleeping, waking, bathroom, washing, any other issues etc. for two nights. It would be a pretty big bloody difference for an adult, never mind a 7 year old.