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.
