Traveling and Inlaws

We are leaving for 4 days, right before Christmas, to visit inlaws and attend a wedding. Now we all know how hard it is to travel with kids, but... I thought that older my son with SPD got, the easier the traveling would be...

well, what has been happening is that when its just the four of us (Dad, Mom, older bro & him) going camping or traveling places where we don't know anybody, he does great. When we travel to see my husbands family (he sees my mom and 2 of my brothers on a regular basis) he hardly speaks around them.

He hides behind me and next me and I answer questions for him. My inlaws are not very understanding (or interested) in why he behaves this way. We decided to stay in a hotel instead of with relatives, we got a nice suite with a fridge (he has a special diet), it has a pool so he can burn off some energy.

Finally, here's my question, what can I do to help him enjoy himself around his grandparents and aunts and uncles? I would love for them to really get to know him and for him to get to know that side of his family. Also, and suggestions on making the 5 hour car ride easier?? By the way he's 10 years old.
Thanks so much!