On his current weight centile (91st) he is likely to hit 18kg somewhere between 3 and 3.5 years old. 98th centile would be even younger - probably under 3 (around 34 months). If he's previously been on 98th and is now 91st, it could be that he is losing some of the baby chub as he gets more mobile or it could be that he is between the 91st/98th centile and moves up and down (this is normal).
Under 3 is an absolute no. You'll find nobody who argues for the safety of a child under 3 in a HBB. They definitely need to be in a 5 point harness. At 15kg he can legally use a HBB now, but this is not safe.
Just turned three - this is pretty young for a HBB, and most people would say too young. The majority of manufacturers put 3.5 as a minimum age for HBB use, some of them have age 4 as a minimum. Some experts would push for a higher age than 4, even. When I used to fit car seats, the store policy where I worked was age 3 minimum but I tended to find that the 3yos were still very wriggly and didn't look right in the HBB seats and I usually recommended a harness for them, even if they met the minimum weight of 15kg. The few 3yos who seemed OK were on the later end of 3, closer to age 4. I'd be really reluctant to recommend a HBB for a younger 3yo, unless it's a scenario like a 15 min taxi to a holiday destination and the alternative is no car seat at all. If you have the option to harness him longer, absolutely go for that.
Seats which are approved to the older safety standard R44 have set weight categories, so they go up to 18kg or 25kg.
There is one forward facing 25kg limit R44 seat - Maxi Cosi Beryl - but it's been discontinued and it's quite hard to find, it also wasn't cheap and it didn't get a very good ADAC/Which score. However, I do think this could work well for you if you can find it, because the frontal crash test score on it was really great for a FF seat. It was the side impact score where they marked it down, and this was because with the 7yo dummy, the head poked out of the top and it crashed into the side door of the test vehicle. Since weight is the issue rather than height, you probably wouldn't run into this problem, and you could stop using the seat once he reached the top of the seat shell by height. However, it doesn't convert into a HBB seat so you would have to buy another seat later.
The old Joie Every Stage (R44) and the Recaro Tian and the Maxi Cosi Tobi are all R44 seats. I do like the Maxi Cosi Tobi, and would recommend it if not for the weight limit issue, though again it doesn't convert to HBB. With the older, belt fitted seats you tend to see that the 123 seats (covering Group 1/2/3, so harness up to 18kg then booster) perform worse in comparison to the Group 1 only seats (up to 18kg, harnessed stage only). Recaro Tian is unusual in that it performs just as well as a Group 1 seat despite being a Group 1/2/3. I would personally not use the Joie Every Stage as a forward facing Group 1 seat as it got a very poor grade in this mode and Which? marked it as a don't buy for this reason.
There were quite a few rear facing R44 seats with the 25kg limit, though none of them are produced any more, because these are mainly made for the Nordic market and R44 is no longer legal to sell in the EU. So dwindling stock if any left for sale in the UK where it is still legal to sell them.
A weird wildcard here - you could get the Britax Multi-Tech 3: https://www.baby-2000.co.uk/britax/britax-multi-techbr-iii/
This one has a forward facing harness mode up to 18kg, and then if you did need to use it after 18kg but he's not old enough/mature enough for the HBB mode you can turn it rear facing and use up to 25kg. It might be a compromise to cope with the travel sickness/plan journeys around sleep times for the 6 months or so until he's a bit older? It can also be used as a forward facing HBB up to 25kg though I have heard people say that this is not ideal since it is a heavy seat. It's seatbelt fitted with tethers like the other 25kg ERF seats - the tethers would need to fit around the seat it's on so you might need to find out if you can get fitting support from the retailer or see if another ERF retailer would offer this. There are some videos on youtube.
The most up to date safety standard is R129 which is height based and the companies are allowed to set their own weight limits, not necessarily adhering to the set boundaries that R44 had. However, there is a hard limit of 105cm height for any seat which is based on that old 18kg weight category. With your son's current stats he is on 75th centile for height and you should be able to get to (just) 4 with a 105cm limit.
The Joie Every Stage R129 got a better score on ADAC compared with the older R44 every stage, but it doesn't have a forward facing harnessed mode. This seat is just rear facing up to 105cm (max 21kg) and then forward facing as a high backed booster from 100cm. (Graco Slimfit R129 is the same).
There aren't many R129 seatbelt fitted seats on the market yet, especially forward facing. Joie have the Elevate R129, which has a 21kg/105cm limit for the harnessed mode, Fortifi R129 (which is only available at Halfords) which has a 22kg/105cm limit on the harness.
Graco which are the "even more budget" arm of Joie have a few too - FlexiGrow which looks like the Fortifi, this is 22kg/105cm limit harness, and Eldura which looks similar to Elevate, this is 21.5kg/105cm limit harness.
None of these have been tested by ADAC as of yet - they do usually release a load of test results in May so you could try waiting a couple of weeks. The Fortifi/FlexiGrow looks very similar in design to the Every Stage, of which the R44 version did badly in forward facing mode, but that doesn't necessarily mean this new model would.
The Eldura/Elevate look more basic and more like the style of 1/2/3 seat which is basically a high back booster with a harness routed through it. This type of 1/2/3 seat has historically not done well at ADAC, though again that may not say anything about this newer one. One thing that is positive is that Joie HBBs and the Joie-made Graco HBBs have done well on ADAC in the past.
The Fortifi and FlexiGrow do seem a bit more substantial looking. The R129 crash test is more stringent than the old R44 one. But without a test result, it's really hard to say for sure.
If there are other brands which have FF, belt fitted R129 seats, I can't easily find them in the UK.
Then you have a lot of rear facing R129 seats (up to 125cm) as PP suggested.
Sorry this is so long and perhaps not very hopeful; there aren't a huge amount of good options to look at. I would encourage you not to bother looking at the ADAC scores of seats which are outside your criteria, there is no point comparing to seats that you can't use anyway. Rear facing is a simple way to gain safety but travel sickness is also a pain to deal with here.