I think I'd look at these options in that situation:
Tinyseats2 as a PP suggested. Pricey though. But could also be used as the main car seat if you wanted to, since the Pearl 360 is outgrown at 105cm/17.5kg. With LO's current stats that is likely to be in about 6 months' time (sorry). Although it is a travel seat, I have seen people say it's comfy enough to use as a main seat. I haven't seen one in person to say whether that's true. The rear facing mode should last you until about age 5 and then it converts to high back booster.
Joie Elevate I think won't last long enough, as the harness will be outgrown too early, but the newer version, Elevate R129, might work as it has a 105cm/22kg harness limit. Graco Eldura R129 seems to be a very similar seat but I am not 100% sure of this.
Or Joie Every Stage R129 (or Graco Slimfit R129) which have the same limits as the Elevate, but are rear facing for the harnessed stage.
Wouldn't recommend either of the Joie/Graco seats as a main seat unless on a vv tight budget as they are quite basic, but they are good for travelling with due to being lightweight, seatbelt fit and low cost. Both of them convert to high back booster for when he is older. The 105cm/22kg limit is likely to last you until about age 4 - it might be a bit dicey if you travel close to Christmas 2025, but should be fine if you tend to travel in summer.
Cozy n Safe Hudson (not i-size) might be worth a look as this is harnessed up to 25kg without height limit, then converts to booster. It needs top tether and isofix to fit, but that isn't a huge issue if you're hiring cars in EU or North American countries as they have had these fitted on new cars as standard for ages now. There are a couple of other 25kg harness seats on the market, like Joie Bold, but the Hudson is the lightest. The R129/i-size version is no good for you though, as the limit on that is 18kg. A 25kg limit should be ample.
If you do choose to hire, I think you'll be alright this year if you go for the 9-18kg or up to 105cm stage as he is well within these parameters currently. (If you are only asked to provide his weight, state it as 14kg.) It will be tricky for next summer as he is likely to be close to the 18kg by then - so you'll get your money's worth if you buy an extended rear facing or extended harness seat now because you'll almost definitely need one next year, and you are coming to the limit on your main seat now too. (It does have an annoyingly low limit).
Although he's over the 15kg minimum for a high backed booster, I wouldn't be comfortable putting a child in one at 2.5 years old outside of a "better than nothing" situation, and he's not tall enough yet for an i-size booster which is minimum 100cm. You want to aim for over age 4 if possible.