T.S.
Hello A., It I could I would like to encourage you to look into the Britax regent booster seat. If is a latch equiped five point harness that goes up to 80 pounds. There is a video on youtube about a boy who was ejected from the car in his graco booster seat because his seatbelt failed. I have one for my daughter who is two, however my 10 yr old sister can fit in it with a strap to go. We all want to make sure our children are as safe as possible, and I think in the car we should take extra precautions to keep them safe. Look into this seat and do some more research. Technically your son could go into a belt positioned booster. He meets the weight limit listed on those type of seats. But give thought to how well you would like him to be restrained if you get in an accident. Also, I had to go to traffic school once.....and learned that if you are traveling 75 miles per hour and get into an accident then you might as well not have your kids in a seat at all because of the impact.
I take car seat safety very seriously and if you have any other questions you would like to ask feel free to send me a message.
Do your research on different seats and state requirements, and definitely look into the Britax regent.....it is worth the peace of mind.
T.