Short of Stature

I would definitely keep him in a 5 pt harness due to his size, and he is only 5 so should not be too bad for him. I would recommend the Graco Nautilus also but there was a big recall recently due to the belt not releasing when you push the button. They recalled them since it could be difficult to remove the child quickly in an emergency. I loved the one we had and we only had an issue with the release once or twice.

I currently have my 2 yr old in an Evenflo seat and it is pretty much the same width. I think it goes to 80 pounds.

My son is too...he is 8 now (turns 9 next week), and while born typical height and weight, had dropped of the charts. He has seen an endocrinologist, that actually put his bone age at 5 years old, so he growing to do!

About 6 months ago we moved him from a 5 point harness to a high back booster seat. He just wasn't big enough, and since he can put himself in and out of a seat, it really wasn't an issue for me. We had the graco nautilus, which I love and while it is bulky, it is narrow. And I understand about siblings, we 4 kids, two in car seats, and both of them are the nautilus and fit right next to each other in the seats really well, and leaves plenty of room in the third seat (all in the middle row of a minivan).

I would not switch him anytime soon (although you didn't mention height/weight), but there are narrow 5 point harness big kid booster seats out there.

Grace nautilus. Love the seat we have two. Used It Two years harnessed only because my daughter out grew it n now it's her booster. I can not say anything bad about that seat.