It is possible but it will be some work, I flew with DD for a 10.5 hr flight, she was in her car seat for most of it, other than changes and all, I think if you are willing to drive at a time he is used to sleeping it might just happen! About the in-laws and their childrearing opinions, do what I do, say oh I haven't tried that yet, but I really don't want to mess up his routine so we can enjoy this time here and not have him cranky! Maybe when I am at home I will give it a try... so just be polite and don't get into their games. I know it's hard but you have to remember this is important for Dh and also for DS to be involved with his extended family! In the end it really is your call,
Good luck !