In Texas the laws changed as of Sept 1.
Children under 8 or less than 4'8" tall have to ride in a booster seat. No where that I have seen does it state it has to be a high back booster. From now until June 10,2010, cops will only give a warning. After that date, the first ticket is $25 (I think) and the next is like $250.
My son has been in a seat only booster for a couple of years now. Friends of mine also said they were told by a cop friend that children riding in a 1/2 ton or more vehicle don't have to ride in a booster seat. I haven't read or heard that anywhere. So, I have come to the conclusion that they just made that up, so they don't have to buy (or make him ride in) a booster for their child. He's the same age as my son, which really bothers me. But, when my son asked, I just told him that it didn't matter what they make their son do, I wanted my son to be as safe as possible.
I see one of the posters suggested calling your doctor office. Another place to call is a car dealership. Certain personnel at dealerships have to attend car seat installation classes on a regular basis.
BTW, also as of Sept 1, all passengers, no matter their age or where they are seated, have to wear a seatbelt in a vehicle.