It's difficult to advise because he's at a really tricky stage to give blind advice - as PP said, it would be helpful to know budget, his height/weight, and whether you prefer rear facing (safer) or forward facing (many people do by this age).
Would also be good to know what the previous seat was and what you liked/disliked about it/what the issue is with it.
I can give it a go anyway because I do love a good car seat search.
Best guess/all rounder
There is a good deal on the Britax Advansafix Pro at the moment on Kiddies' Kingdom, £149 where it is usually over £200. That one has good safety (for FF) and can use the car seat's own straps up to 105cm/21kg then converts to high back booster. It has to be fitted using isofix and top tether.
If you prefer to see/try a seat in person before buying (which is a good idea), John Lewis have the Silver Cross Balance Essential for £159 which is similar in stats and also a good option.
If he's likely to outgrow a 105cm/22kg limit before age 4
You could look at Cozy n Safe Hudson/Excalibur - not the i-size versions, though. These two seats have a 25kg harness with no height limit. Safety is less clear than the Britax/Silver Cross, so would recommend those other brands if you don't need the extra weight/height capacity. This also needs top tether. While these seats are cheaper than the Britax/Silver Cross, if you need cheap but not the extra height/weight limit, I'd advise rear facing instead (see below)
If you need something cheaper
Joie Stages is £99 and can be used rear facing up to 18kg and approx 115cm but doesn't have a height limit. This is probably the safest seat you'll get on a budget due to the rear facing. I am not hugely keen on this used forward facing, but if fitted correctly, it should be safe. It can extend into high back booster mode up to approx 125cm, max weight 25kg. Ideally, use as a RF seat and then replace with a high back booster, because the HBB mode doesn't last long enough, so it will need replacing anyway, but it's not terrible to keep as a spare or use infrequently.
Or Graco Extend LX is £90, similar and meets the new safety standard, so may have better forward facing safety? Should be similar rear facing. The trade off is that this has no high back booster mode at all and is outgrown at 105cm/18kg entirely. Both the Joie and Graco seats do have an annoying belt routing in rear facing mode.
Graco Slimfit R129 may also be worth a look - £109 at Halfords. Rear faces up to 105cm/21kg then can be used as high back booster 100-150cm. Reasonable safety rating from ADAC, and the latest safety standard but has NO forward facing 5-point harness mode at all.
If you want ERF
There are rear facing seats up to approx 125cm (7 years ish) which start from about £170-190 if you are keen on rear facing and can afford to spend a bit more. They don't need top tether or isofix and can accommodate children who are bigger for their age. Top safety - so worth considering.