Car seat!
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5 point harness forward facing isn’t safe at any age as it holds their body back but their head would go flying forward and can internally decapitate him. It sounds as if he’s not mature/old enough for a HBB. usually this is closer to age 5 than age 4. He needs to be rear facing. My son was 4 in august and his rear facing seat is expected to last till he’s about 7-8 years! Rear facing is the safest way to travel (even for adults but obviously that isn’t realistic) Children’s bones aren’t fully developed / ossified so at such young ages they should still be rear facing until they are old enough for a HBB which again is usually minimum age 5 x
My daughter is 4 and has the same seat as your son. You can buy something that the top belt and bottom belt clip into and it is to lower the top belt but it will also keep it more secure. Message me if you are unsure and I can send you a picture of the one I got my daughter
Does he have good understanding? Id explain that he must sit sensibly and leave the seatbelt alone whilst you are in the car and explain gently what could happen if you were to stop suddenly and he not be in his seatbelt properly?? We’ve had to explain to my child not to unbuckle until a grown up has told him and what could happen if he’s not strapped in properly as he was getting out of his straps. After the gentle chat he hasn’t done it again. my son is 5 in jan 110cm tall and we still rear face. x
May not be old enough for that type of seat if he can’t sit properly in it. If you post his height and weight someone can advise which seat will be better. Other than that boundaries he doesn’t travel in the car unless he can sit properly xx