Hennightpanic I do not think you have done anything wrong and I do not think you need to be more welcoming.
Do you know him?
Have you met him?
Do you want him in your house?
If not, I would spell it out, "I am very sorry but you will not be able to stay here. My son has special needs and I have to keep things very calm and even for him at home. We were delighted when XX (name of friend) wanted to come and stay as we know she will be with XX (name of your dd) ... and presumably your son knows this other teenager so it is not a stranger for your son.
It is not how much space this man needs, it is whether you want an uninvited adult in your house. Do you have a dh/dp? How do they feel?
If you genuinely have space and don't mind that is fine, but if you would not be happy with him tagging along with you and your son or hanging around your home, and staying at your house, then you need to be very clear with him, because the comment "he's flexible and doesn't need much space" sounds like a massive hint!
I agree with pillowaddict when they say Explain now that there seems to be a bit of a misunderstanding I would not phrase it as just space, although by all means say this too if it helps, but I would say it is about your son and his routine etc. It's not that unusual to have children who need, calm, space, not to have strangers in the house. My kids would find it very hard to have a new person staying for days, a week??? when they had never met that person
I would also say what you can offer, a day out altogether at the start or the end, if you want to and a meal etc when you feel it is right.
And yes, agree to suggestions of somewhere local he can stay when he is not exploring the country independently as he first said he would.