Do you have an idea of budget at all? There are quite a few ERF seats which would work for him long term.
Axkid Move, Axkid Minikid 2 or the Britax Max Space Pro (if you can find any still discounted) tend to be the cheapest options which last a long time. They can be set with a bit of a recline and less leg room, or less recline and more leg room, so adapt well as the child grows. These are around £250. Axkid Move is the cheapest, but it's not much more for the other two and they do have slightly nicer features. The two Axkids are R44 certified, older regulation but still fine. Your DS will need to be over 80cm tall to fit comfortably in the Move, a couple of centimetres smaller tends to be OK for this Minikid.
The Britax is the newer, R129 regulation, certified for 60-125cm with a weight limit of 36kg (which is essentially - limitless as it is very unlikely to hit that before the 125cm limit.)
All seats last up to approx 125cm (The Max Space Pro can fit a physically taller child in, but is still limited to 125cm legally.) and the weight limit on the Axkids is 25kg, which should get your DS to around 4.5 - 5 years old assuming he stays on the exact same centile (and it's actually not that uncommon that once they start moving around, they drop weight centiles anyway.)
The next jump up in price, ignoring a couple of models in the middle which don't have any significant benefit (to you) over those three seats, puts you at about £400-500. The one that I think is worth a look in this price bracket is Axkid One2, mainly because it's currently got a decent discount on it. Unlike most seats in this category, it's isofix, which means you can change the recline without uninstalling the seat, though the downside of this is that they have a slightly lower weight limit at 23kg - but you do have the Joie Bold, so you could keep hold of this and use that for the last two kg if this turns out to be a problem.
The other seat you may wish to look at is Besafe Beyond, because it rotates, but given you have to buy the base as well, the total cost is over £600 which makes it not worth it for most people.