Harness car seat for bigger toddler

My cousin has a toddler that will be 4 later this year. He is quite large for his age (around 80 lbs) and asked me what car seat we use for my now 4yo. I looked at my manual and it said for children under 4 years old but over 65 lbs to discontinue use of the seat. I cannot find car seats anywhere that will fit a child larger than 65lbs without it being just a high back booster. My cousins child is really still far too young to be in a high back booster, but can’t fit safely in a forward facing harness seat. I am so concerned for him but i can’t find one anywhere. Please if you know of any, let me know