Baby's shoulders too tall to fit in Convertible carseat

Hello everyone, I'm a first time mom and I've run into an odd situation. My son Greyson is a year old and weighs almost 30 lbs. His pediatrician has always told us he's going to be tall for his age. I got him a convertible carseat when he was born thinking it would be easier and being a big Baby for his age he quickly grew into it. A few weeks ago I had to adjust the seat and the harness is as far as it will go. It's in the top notch but now I can hardly get his arms through the straps it's such a tight fit. He's still back facing and in the reclining stage. His seat would be fine if it wasn't for the harness being so tight. I wanted to see if anyone has any suggestions on a carseat for a taller Baby that has more than 4 slits so we won't have so much trouble fitting his arms in and he'll be more comfortable?