High backed boosters approved under the newer R129 standard do not need to have a weight limit, only a height limit. So there ought to be several R129 boosters with no weight limit. In practice, many of them seem to have a weight limit - 36kg like the old standard, or something else.
With the older R44 standard, 36kg was the end of that standard, and it was not possible to certify booster seats over this weight, but that doesn't mean they can't be used over that weight. You must obey the weight limits with a harness, because it may fail with higher loads, but with a booster seat you can use your own discretion to go over it - the adult seatbelt is tested up to something like 200kg body weight. There used to be some official sources confirming that it's safe to continue using a booster seat over 36kg, as long as the child physically fits into it, however because the newer regulation removed this restriction, you don't tend to see this advice "officially" any more - they just point to the newer regulation.
The fact that the newer regulation does not force companies to specify a weight limit makes it clear (IMO) that a max weight for booster seats doesn't really make sense.
As for why some companies have continued to apply the 36kg (or other limits like 50kg) weight limit to their R129 boosters - I have no idea. It might be just for consistency with older models. It might be that they actually don't recommend its use after that weight, for some reason. It might be that they consider something like the side impact protection is sub-optimal after a certain weight (which you could of course ignore, because if the alternative is no seat, then the side impact protection is non existent in that scenario anyway). It might be that they have looked at a weight chart and based it on age. You could write to the companies and ask for clarification, or you could make a reasoned decision that having the belt adjustment for the hips and shoulder is more important than the seat working exactly as the manufacturer intends.
If I were you I would forget about weight limits, but look at the belt fit on your son, this can be less optimal with children who are on a higher weight than height centile, especially if they are approaching or over the limit of the seat, and it can be worth changing seat in order to get a better belt fit if this is currently not good, because belt fit is the most important job of a HBB. Lap belt should run over the tops of the thighs, preferably in contact with the pelvic bone. If it is over the soft abdomen, it is not a good fit. Diagonal belt should run centrally on the shoulder, not over the shoulder tip, but also not touching the neck. It should cross the collar bone.
The child should be able to sit comfortably without slouching, with their bum to the back of the seat. If they are slouching because their feet almost reach the floor, you might want to add a foot rest such as an empty cardboard box, suitcase, or you can buy inflatable footrests or ones which click into the isofix under the car seat.
Diono HBB seats tend to give a good fit to children with this body type, as they are a bit roomier. They are all R44, I think, so all have the 36kg "limit" but they have the exact same models approved to US standards with a much higher weight limit of 120lbs (about 54kg) They are sometimes cheap on amazon, though currently don't seem to be. One colour is £125.
I agree that Cybex can be good too - I sat in a friend's car to secure my DC in their middle seat and they had a Cybex HBB on the side - I could sit on it quite comfortably with adult-sized hips, though it would have been too small for my shoulders!