I know this is a bit late for what you were asking - but both my boys had a problem being comfortable in their booster seats by age 7.5 or 8. I wanted them to stay in them as long as possible, and the laws in SC and NC (we've lived in both) say they have to be at least 8 yrs old OR weigh 80 lbs, but our issue was finding a comfortable one. They had the high-back most cushiony things I could find, but they hurt the backs of their legs below their knees. So i switched to just a seat booster, tried two different kinds, and they had the same problem. For a while I tried rolling a towel to prop their legs on, but that always lost its shape... Finally after talking to our ped., he said if they can sit straight up, the shoulder belt fits correctly across chest, AND their legs can bend at the knee, they can sit properly enough for a regular belt. With my older son, that was right then - he was tall enough (or rather his legs were tall enough!). When my second one reached that age, his little legs were having the same problem, but he was too short for a while, so we kept doing the rolled up towel thing. He just had to put up with it for a while until his legs grew longer.
My daughter (almost 9) LOVED hers, but finally outgrew it. She is one of those that won't be 80 lbs until age 12! I think the boys did feel like "babies" and just want to be big enough, but I wouldn't do it unless they can sit safely. Sometimes I "threaten" to pull ours back out for my dtr, b/c she will lay down across the seat, or slouch too much and the belt goes right across her tummy.
So watch your son for signs that he is tall enough, and if he becomes uncomfortable in the seat, try towels/blankets or seatbelt adjusters. He will understand that you are only concerned about him being safe. I read an article once about car seat safety, concerning people that take their babies out briefly, or put kids on their laps for short trips (yes, there are some idiots who still do that!), but it said to think every time you get in the car that THIS IS going to be the trip that you are in an accident, and how would you and your family want to be strapped in if you knew another car was going to run into you?! Pretty much straightens out your feelings!
My best to you!
A.