I wouldn't put him into a high back booster at his current age and height.
You do have a few options however.
The In Car Safety Centre have some of the Klippan ERF seats up to 25kg on clearance for £150/£125 - if you contact them they can let you know which are most suitable.
As FamilyAreEverything said, the Britax ERF seats are also on offer with various retailers.
Any of these (Klippan/Britax/Axkid) ERF seats would last until around 5-7 years depending on the weight limit.
Joie Bold would be worth a look also for £150ish if you can find it in stock - it has to be the old, R44 Bold (or Bold R), not the newer R129 i-Bold. The original one has a 25kg harness limit with no height limit on it and then converts to high back booster. It's forward facing only. Discontinued, so can be difficult to find stock but it does seem to still be around in a few places.
If that is out of budget you could look at Cozy n Safe Hudson, Cozy n Safe Excalibur or Puggle Safe Fit & Grow which are each around the £100 mark - again, avoid the i-size versions. They are 25kg harness without height limit, convert to high back booster, FF only.
A 25kg limit FF seat will last him to about age 5 in the harness, but they do then convert to HBB, so in theory you won't need to buy another seat.
You could also look at the Britax Advansafix or Evolvafix seats, or Silver Cross Balance i-size, these are forward facing too, all have a harness limit of 105cm/21-22kg and then convert to high back booster, they are better in terms of overall safety than the 25kg limit ones, especially in the high back booster stage (which you'll use much longer), but the harnessed stage won't last him as long - likely around 6-8 months from now, if you go by height limit, or somewhere between age 4-4.5 if you are happy to overlook the height limit and take it to the weight limit (personally I'd be OK with that if it's only a centimetre or so).
Anoris I think is not really worth it, because it will only last you a year or so, the weight limit is 21kg as well (115cm height limit), doesn't convert to HBB and it's over £400 even for the older model.
You might have been looking at the Cybex Pallas models which use the impact bar up to 18kg (or 105cm/21kg for one model) and then convert to high back booster, but these don't really have an advantage over the 5-point harness seats, because they don't have built-in airbags (like the Anoris).
So in summary -
If you prefer RF you could look at the ERF offers at the moment under £200 - Klippan or Britax. Definitely worth it if you plan to RF for the long haul.
FF options:
Joie Bold - longer lasting harness up to approx age 5.
Cozy n Safe Hudson - budget choice with harness up to approx 5.
Britax/Silver Cross - better overall safety, but might only get ~6-12 months from the 5 point harness.