My daughter, for her 7th birthday, had a sleep over at our home. 4 other girls were invited.
It was with, her GOOD friends, which I also know well and know the parents well. They have come to my home, many times before.
So it was fine.
We are all, pretty much the same way, as parents.
I really don't know what you should do.
You can tell the parent your son has never stayed anywhere before.
You can ask about supervision at the party.
Who will be there etc.
What is the activities etc.
Any "Host" of a sleep over, would understand these questions and gladly answer them for you.
IF the Mom, does not respond very well to your questions/concerns, then I would opt out.
If the Mom is flippant about your concerns, I would opt out.
If the Mom, seems irresponsible, I would opt out.
BUT, you need to understand, going to a sleep over means:
The children will do things, that may not be, what your son does.
Even bedtimes will be different.
Foods/snacks may be different.
Supervision may be different.
etc.
etc.
etc.
This is the nature of a sleep over.
When my daughter had her sleep over... they slept very late. I'm talking very late. Midnight.
They were like chirping birds all night. Chatting.
They are good girls.
I am a good Mom who supervised everything.
I let them have fun.
I MADE sure, the other Moms knew... they would most probably sleep late. They were fine with that. It is a sleep over. Kids are active.
None of the Moms, had a problem with the sleep over. They know my daughter and I, and trust us.
It is an exception, because it is a sleep over... that kids can sleep a little later, eat snacks, and be silly. Which girls are and do.
It was fine.
But in your case, you do not know the Mom very well.
So I don't know.
Also, older cousins will be there, too.
But you cannot, control everything, at someone else's party.
So again, you should talk to the Mom, hosting this sleep over.
See, if she is okay... in answering your questions.
Use your instincts, and common sense.