@Hei3htfi
You've said he has consistently stayed around the 75th–90th percentiles his whole life, so his current build is healthy and genetically predetermined, don't worry. He is still growing!
Currently your son is on the 91st percentile for height and 85th for his weight. Concern arises when a child's weight percentile is significantly higher than their height percentile which his is not.
A more accurate tool, to address your concerns, would be to use the Waist-to-Height Ratio (WHtR)
The Rule: Keep the child's waist circumference to less than half of their height.
The Target: For a height of 136 cm, his waist circumference should be under 68 cm.
Clinical studies show WHtR is an inexpensive, highly accurate tool for measuring centralized fat tissue rather than muscle mass, and it correlates directly with metabolic health.
If, in the future you'd like to approach his diet with portion control then fill half of his plate with non-starchy vegetables or fruits, one-quarter with lean protein (chicken, beans, fish), and one-quarter with whole grains. If he is still hungry, give seconds of the veggies or whichever protein you're having.
As someone also said upthread, you could downsize your dinner plates
This naturally reduces caloric density while maintaining high volume and fibre to keep him full.
; for sp.