What kind of seat does he have at the moment? I'm assuming a convertible that can rear and forward face?
Rear-facing is always the safest option for a child of any age, but unfortunately the highest rear-facing seat I know only goes up to 44". This is the Diono Radian RXT. This would be ideal for your tall son. It holds from 5-45lb and from 19-44" rear-facing, and then forward facing from 20-80lb and 57" in the harness, and then a booster up to 120lb and 57". My son just turned forward-facing and he is 6.5 years old. He is small though. Other options are The First Years True Fit C160, Graco MyRide 65, Akta Graco DuoFlex, Britax Multi Tech, Britax Boulevard, Britax Marathon and many others.
If you do not want to rear-face him (which would be selfish on your part as it is WAY safer) you can purchase a forward facing toddler seat. These only forward-face, and typically hold from 20-65lb and up to around 52".
The Graco Nautilus holds from 20-65lb in the harness, 30-100lb as a booster and then from 40-100lb as a half booster. It also holds from 27-57".