Per the rules your son has a STR of 25 as he can lift a weight of 750 lbs and Hafthor Bjornsson str would be 36 or 37 as he can lift 1100.
Those are 5th edition stats and they are impractical. According to 5th edition the average person can carry 150 pounds. That means a person with a 10 strength should be able to carry a full combat load (35 pounds), plus all the kit that an airborne trooper carries (about 70 pounds), and even that of a "Ranger" load out (which is generally about 100 pounds of gear). And yet Rangers are constantly training to keep their physical fitness at peak levels (or at least a 15, but more like an 18).
Based on the 15 x ability score, a person who can dead lift 750 pounds has a strength of 50. A bench press of 350 is a Str of 23 for the 5th edition rules.
18/00 is the maximum human body lifting capability, period. A human body can not have a 19 or better strength the physics of the body make it impossible (according to 1st edition rules).
You move 70% of your body weight in a push up. Average height for a white male in the US is 70 inches. and the average weight is 165 pounds, so the average person could move 115 pounds, a strength of 8. We know this to be too low, as such the mathematics of 5th edition is flawed.
In 1st edition there is a table starting at strength of 12 there is a"'weight allowance" of 10 pounds, this increases to 300 pounds at 18/00. This is the amount of weight a person can carry without becoming fatigued, or moving at a slower movement than the 'base rate" of 120 feet per turn.
5th edition, in an effort to simplify the game, made the strength modifier so high so as to make encumbrance a non factor. I quote:
"your carrying capacity is 15 times your strength score, which is high enough that most characters don't usually have to worry about it."
So, the numbers in 5th edition are just plain impractical.
Better to use the 1st edition, or even 3rd edition which gives an 18 strength a 300 pound weight allowance, and an 11 of up to 38 pounds for light, capping out at 115 pounds for heavy. This would then be the 'minimum" strength for an Army Ranger, with a 15 or 16 ~ 70 pounds as "light" and 133-153 as medium, capping out at 200-230 pounds as heavy.
figure 35 pound combat load, 35 pound ruck, 30 pounds of additional gear (like a radio), and we are 100 pounds, and then you have, for a mortar team, a 50 pound base plate, and ammo, mortar tube and ammo, etc. M2 MAchine gun team 50 pound lower receiver, 35 pound barrel, and a 35 pound tripod, plus ammo...
