The Honda should lock. The only Honda I've been in that doesn't have a Switchable Retractor my FIL's 1992 accord. It became manditory starting with the 1996 model year but I'm nearly positive that Honda was compliant before that.
The Buick, unless pre 1996, should also have some sort of locking mechanism. I know my grandmothers Century does and so does my dad's LaCross.
Teach your DS to sleep with his head to the side, using the head supports. Or slightly recline the seat.
Asleep in the back seat, he'd likely fly out the front windsheild in a crash.
In college my 20 year old roomate was asleep in the backseat of a car that was in a hwy collision. She was thrown out of the car about 50 yards. She was luckly, she only suffered a broken pelvis and broken arm. My friend Chris wasn't so lucky. He flew through the sunroof and landed about 25 yards away. They think he died instantly.
ETA
http://www.britax.co.uk/ has videos of unrestrained child crash test dummies. They are kinda hard to find but if you click 'think campaign' on the left hand nav bar and then GMTV test videos on the think campaign page you should get to them.
Here are the direct links, the tests simulate at 30mph crash
http://www.britax.co.uk/movies/Brit...trained_Child_Facing_Forwards_in-car_view.mpg
http://www.britax.co.uk/movies/Britax_GMTV_Check_It_Fits_Unrestrained_Child_Facing_Forwards.mpg
There are 2 more but they relate to infants in lap and old/used restraints.