They are weird... so avoid them.
Listen to your 'radar' or gut instinct.
Frankly... I would not want 'my' kid IN their home... unless I was there too... at all times.
And I would not send my kid there, to play, by himself.
Don't just do it, just because you want your son to have someone to play with.
By now, at his age, your son must have his own schoolfriends... so just invite THEM over to your home.
Your son... is also... old enough for you to explain to him.... about this.
My Daughter is 8 years old... and even when she was 7 or so... I would just talk openly/honestly with her... if she asked 'why' I didn't care for another parent or did not 'trust' them etc. I explained these things to her and why... because she was at the age where she could understand these matters, and would benefit from it being explained to her. It widens the level of their understanding... per people and what is appropriate or not. My own Dad, when we were kids... would ALWAYS explain things like this to us from when we were young. He was a good judge of character and had good instincts. Sometimes, these things too, needs to be taught... to a child, from a certain age.
Tell your son openly and honestly. He is 9.
Teach him, these things... about how to assess... others....
all the best,
Susan