To get a rough idea, what size clothes is he in? Most 123 seats which have a harness up to 18kg will fit children until they are too big for age 3-4 clothes, particularly in tops rather than trousers. (Obviously, that is an approximate measure!) The vast majority will outgrow this kind of harness in height before outgrowing it by weight. A harnessed seat is outgrown when the child's shoulders are level with the top harness slots, because in forward facing the harness must come from above or level with the shoulders, never below. NB, these seats are safe if the child is within both the height and weight limit for the harness. There is usually a limit on how high the harness can go, whereas the headrest can go up higher for use in highback booster mode.
There are a few seats on the market - Joie, for sure - where the harness can be extended all the way up to the top height when the seat is in high back booster mode. So in theory they can be used right up to the 18kg limit regardless of height. You get a similar impression from impact shield type seats, as they don't have a harness so there seems to be no maximum height. But - I'm a bit sceptical about all of these as options. The Joie Every Stage gets bad results in crash testing when forward facing. The Joie Stages has not been through independent testing. The Joie Elevate is apparently difficult to get a secure fit with in a lot of cars (but is cheap) and impact shield seats seem to perform more poorly the taller/older the child in them (and aren't great to begin with). It might be that past a certain height the top tether is really instrumental in protecting the child's head from coming too far forwards in a collision which is why all of the 25kg seats need top tether and why most of the 18kg harnesses are height limited. (It may also be why some of Britax's extended harness/ERF seats have changed their classification in the newer models.
It might be worth, if you think you might have another 6 months or so in 3-4 clothes at least, taking him to a shop like Mothercare or John Lewis and seeing if you can try him out in a few different Group 123 seats to see how high the harness goes and also seeing how well they fit in your car. Halfords tend to be really poor at fitting advice, so I'd avoid them. Or you could watch some fitting videos on youtube and have a go yourself. The other way to do it would be to measure his floor-to-shoulder height when he's sitting on the floor straight up against a wall, and comparing that to the top harness height of various seats in a shop.
As Tea says you could get more harness time by turning him back rear facing which a lot of children find quite fun. Add a mirror if supervision is an issue. You can get rear facing seats into small cars surprisingly easily, so don't discount them on that basis, and they aren't even that expensive any more. These are all seatbelt and lower tethers, and lower tethers don't need specific anchor points, they usually just attach to some part of the front seat.