Yes, he is too big for the harness mode on his seat.
Based on what you say the limits are, I'm guessing he is in one of those 3 in 1 seats? The booster mode on those has a notoriously bad fit on most children, placing the lap belt too high and making the shoulder belt gap away from the chest.
If you believe that your child is mature enough to sit properly in the booster (no leaning to grab a toy, wiggling around, putting shoulder belt behind back, etc..), than I would suggest getting a dedicated booster like a high back Graco Turbobooster.
If your child is not mature enough to sit properly 100% of the time, I highly suggest a combination seat that harnesses to a higher weight then becomes a booster. Depending on your budget, some good seats to look at are the Harmony Defender 360, Graco Nautilus or Britax Frontier 85.
Here is a good site that shows how a booster should fit:
And this link shows what the best fitting and worst fitting boosters are