If the truck is new enough to have airbags, and it is a regular cab or an extended cab, then it should have a switch to turn off the airbag. Does the truck have shoulder belts? If not, I know you aren't supposed to use a lap belt with the booster (Check the NHTSA website about car seats)
Are you sure your ex is aware of these issues? It took me quite a bit of digging to find out what the car seat laws are in my state, in some of the US states the law is only 3 years, a lot of states have moved to 60 lbs and 6 years but this is fairly recent. And from reading the above post, I guess there are states that go up higher than that. I have a neighbor whose is a deputy and he wasn't even sure if our state had gone to 60lbs and 6 years old
If you want to see the laws by state, you can go to
http://www.hwysafety.orgWhile I think safety is extremely important, I also think you need to keep things a little in perspective, it wasn't that long ago that mother's brought babies home in their arms in the car. Of course today that would never happen.
10 years ago, you would probably never even thought twice about a 4 year old not being in a booster seat - probably even just 5 years ago.
In 5 more years people are going to probably think horrible things about people that have their child in a car that isn't equiped with the LATCH system.