There aren't any seats on the market any more which can do this, and there probably won't be in future, because the safety requirements have increased to the point it is too hard to pass with just a lap belt securing. And if the lap belts are retractable, which is quite likely, then they aren't suitable to fit car seats with anyway.
So no real "perfect" solution possible - a few maybe "least worst" solutions:
You could get a second hand Britax Eclipse or old Britax Prince or Kiddy Infinity or Britax Two Way Elite. (None of these are sold new any more, but all fit with lap belt. The TWE would not be a proper fitting as no tethers).
You could just use the seatbelt, even though he is likely too small to get a good fit - it should be better than nothing.
You could get a booster seat like Trunki or Bubble Bum, even though he's under the weight, for the sole purpose of routing the belt correctly.
You could hold him on your lap (NOT under your seatbelt). This is probably what you are advised/expected to do and is the official fallback for children under 3 in emergencies in the UK when a car seat is not available but the travel is unavoidable and unplanned and as short as possible.
You could get a CARES harness for the plane, and use this on the coach (it's not approved for cars - would be simply to stop him climbing around).
You could get a Ride Safer Travel Vest (approved to US regulations).
You could get a rear facing seat like Graco Extend LX and fit it using the lap belt and wedge it in behind the seat in front, with the aim that the brace replaces the shoulder belt function.