Don't remove him from a harness until age 4. I used high back boosters ( a must if you have shoulder belts or no head rests) but my kids weren't safe in them. They would move the seatbelt, slump down into the seat, ect. So I looked into it and found the Britax Regent. My 4 and 5 YO daughters were in it when we had a roll over accident, and I do believe that the seats saved their lives! I don't believe that boosters would have protected them in that kind of accident.
We replaced them with the Britax Frontier which goes to 80 lbs in harness and 100 lbs in booster! Well worth the money, IMO!! And I asked them what the 'expiration' on the seat was to make sure that my 2 YO could use her new seat for a good long time. They said it was 9 years. Yeah!
S., mom to a 6 YO, 5 YO, almost 4 YO, almost 2 YO and expecting a new one in a few weeks! I've used SOOOO MANY different car seats and boosters you wouldn't believe! LOL