The new Infasecure Comfi Caprice has a harness height of 42cm with padding, 44 without. It's the highest height markers out at the moment. Can you measure you son's seated height? (Bum to shoulder up against a wall). You can also harness passed the turn to booster line to the top slots as it has been safely tested with the top slots with a 32kg dummy.
Your son HAS to be in a 5 point harness until he is at least 4. H harnesses are illegal under the age of 4 and shouldn't be used with a full seatbelt, it's dangerous and a seatbelt is far safer.