I got taken on several nice vacations by my BFF's family when I was a kid, including DLR. Our moms were good friends, and her family knew that they had a lot more money than my family. Her family paid for everything (except spending money). I probably wouldn't have been able to go if my parents had to pay, and the other family knew it and offered to pay. VERY generous and very appreciated!!
I think it depends on your relationship with the family, and it's ok to ask them to pay for a portion if you explain it up front. If it were for a non-Disney trip (no park ticket) that we were driving on, we would just pay for everything (hotel, food, etc).
Or if I could afford it easily, I might just look to pay, but if it's a stretch than I think it's ok to discuss this with the parents. For Disney, I might approach the parents and explain, we would love to have your son join us. We would be able to pay for the food, hotel, etc., if you were able/willing to pay for the park pass (or air fare, etc.) and spending money. I think whatever your offer is , is ok as long as it is clear from the start.
I think it is important for you to speak directly to the parents first, not your son to their son, because if they dont want him to go, it's better handled between adults.