My son is about to turn 3 and tips the scales at 20kg. So according to all of the car seat 'rules' he is too heavy for a 5 point harness, but he just seems too young to put in a high back booster with a seat belt. I had ordered a Joie Bold (harness up to 25kg) but it requires seat belt, isofix and top tether fitting, and we don't have a top tether fitting, so that is having to go back. Short of buying a new car (which we can't afford), i don't know what my options are. Does anyone have a 3 year old in a high back booster with seatbelt. I just think he needs to be restrained, and i don't see how he wouldn't slip to one side in a high impact crash? any experience/thoughts would be appreciated.