He's technically the minimum weight and height to use the booster systems, Mifold, Trunki Boostapak or Bubble Bum, but he's very little and I'd be concerned that the belt wouldn't sit properly on his shoulder with these seats. Possibly it depends how much driving you are likely to be doing, what the roads/driving style is like where you're going to and whether it would be a "better than nothing" kind of scenario (they would almost certainly be better than nothing). If you go for one of these, my preference is Trunki, but it is bulkier than the other two.
You might also want to look at Chicco Fold & Go? This is another booster type seat, but it's at least a high backed booster. I wouldn't normally say that a 3yo only just at the minimum limits should use this type of seat, but again, if your options are that or a backless booster, it's the better option in that scenario. This gets a decent crash test result (Which? or ADAC)
Or there are a couple of Group 1 solutions you could look at - Maxi Cosi Nomad is a harness based seat, and folds for easy transport. Won't fit him for much longer, but you could use for your other child later on too. Up to 18kg/approx 105cm. No crash test result on this. It is a similar design to Urban Kanga which got quite a poor crash test result (and isn't any cheaper so I wouldn't bother with it) - the Maxi Cosi does look a bit more substantial and higher quality, based on nothing other than comparing the fitting videos, but don't know whether it's enough to make a difference. Obviously, portability and sturdiness are at odds with each other, you have to compromise some way or another.
If you prefer rear facing, Nachfolger is a pricey but practical solution, it's a fully inflatable seat that folds down into a small package. Rear facing, though requires either top tether (usually present in hire cars) or a front seat with disabled airbag, depending on the model - some are top tether only. Again up to 18kg/approx 105cm.
Or you could look at ultra-cheap supermarket/Argos type seats like the Nania or Harmony models, these are pretty flimsy and again wouldn't recommend at home, but actually they get better crash test results than the Urban Kanga and might be worth a look for travel use again based on the concept that something is better than nothing.
Lastly there are seatbelt fitted lightweight models from Joie or Britax in the Group 1 stage, which might be suitable for travel use if you want some established safety with the downside of the seat being larger and heavier to transport. Transport tips with these are to fully extend the safety harness and use this as a handle, or put the car seat in a travel bag which can be worn as a backpack.