I guarantee his BF is over 20%!
At 310lbs, over 20% bf would be 70lbs. That would put him around 240lbs on stage, 25lbs less than his last show.
At 290lbs, with over 20% bf, he'd be around 220lbs on stage, 45lbs less than his last show.
Those are bold claims to make when considering the math. Could it be that you're taking into consideration what 20% bodyfat looks like on someone with a normal BMI?