What is he in now? If you prefer him in a harness then I would keep him in his current seat because it is likely it will fit for just as long as any which have a 5 point harness that continue, and because when you do buy a seat, it is better in terms of safety to buy a seat that does one job (ie, is just a high backed booster) rather than doing two (harnessed seat converting to high backed booster).
You say he is only just over 100cm so he is just right at the bare minimum to use a HBB. He is tall enough but only just. I would keep him out of one for now if you have the option. It's likely his seat is up to 105cm (that is the usual limit) so he will have likely most of a year left in it unless he is taller in his body than his legs, or goes over the weight limit, or you have a seat with a smaller-than-average shell.
Do you know his weight?
I can make some suggestions anyway but some of them might be inappropriate so bear that in mind.
To use a harness as long as possible, if you'd prefer that - Joie Bold. You probably need Joie Bold R. Don't wait for a sale on this - it's discontinued so you need to get it now as they won't get any more stock. You need the older version (not the i-size) because this is the version with 25kg/no height limit on the harnessed mode.
The Joie Bold safety rating is only average, so only choose that if the extended harness is more important than the overall safety rating.
Cozy n Safe Hudson is the same story - harnessed to 25kg, no height limit, average/unknown safety rating.
To use a harness for probably the next 6-12 months with a good safety rating,
Britax Evolvafix or Advansafix Pro - both 105cm / 22kg limit on harness.
Maxi Cosi Titan i-size or Titan Pro i-size - 105cm/18kg limit on harness.
Silver Cross Balance i-size - 105cm/21kg limit on harness.
None of these are discontinued so many all go on sale.
For best safety rating, go for just a high backed booster, but there is some good data from US showing that children 15kg+ don't do as well in high backed boosters, children 18kg+ do much better. In the UK it is tricky to get over the 18kg mark without buying a 25kg limit seat, so we just say when moving from the harnessed stage (often 105cm and/or 18kg limit) to the high backed booster stage (100cm and 15kg minimum) try to aim for as close to 105cm and/or 18kg as you can, rather than switching over as soon as you hit the minimums.
Any HBB from Britax, Cybex, Recaro, Avionaut, Nuna, Joie (and the Joie clones so Halfords Transfix / Graco Eversure i-size) are good. There are a lot of good HBBs on the market, whereas to find a good combination seat is more difficult. Your best move really is to go to any big shop, ideally an independent, if it has to be a chain, John Lewis or Mamas and Papas are best. Try them out. See what your son finds comfortable. Ask the staff which ones give him the best belt fit. Check how far the headrest extends up. Ideally try it in your car, check it fits nicely with the seat, check the seatbelt runs smoothly and doesn't get caught and it's easy to buckle up.
If you want to look up HBB safety ratings, type in the name and ADAC and if it comes up with less than a 2.0 for safety (sicherheit) then it's good. Not all models have been tested though.