Hi Sarah! I truly understand your trepidation about this upcoming trip. I've made several trips, many overseas, and it always upped my stress level to worry about how the kids would handle it. And the truth is, he probably won't sleep as good as he does at home. But, once you arrive at your family's house, use a couple of spare towels (the bigger the better) to line the bottom and I would place it fairly close to where you'll be sleeping in case he does awaken in the night - he will see you and maybe go back to sleep.
It probably won't be as bad as you think it might; just bring his 'comfy' things. His body will take over - he will still need sleep; probably more so as you will likely be doing a lot of activities during the day, being out in the sun, being spoiled by relatives! :) It will work out, just know that you'll have to be flexible on the trip and you'll have a lot of help around you - which is great.
Take care and have fun!
L.