I think as long as you make your offer clear, you are ok. For example, if you can't cover the whole thing, just say...
"We'd love to have Joey come along with us on our Disney trip. We can provide the room and ______________."
You just have to spell it out beforehand.
I assume, though, that since this kid is a close friend of your son's, that you know the family fairly well. If that is the case, you might be able to guage what they would be willing (if anything) to contribute to the trip. Or you can just be direct.
For us, I am good friends with all of the parents of my kids' friends, so it would be pretty easy for me to say , "Hey I'd love for Sophie to come with us, but we can't really swing all of the expenses. However, we can definitely cover A,B,and C if you can cover X, Y, and Z." But again, this takes having a solid friendship with the parents.
Just make it very clear what you are offering.