> At the weight he usually hovers at, his BMR appears to be ~2,500 calories. He can lose weight from here no problem eating ~1600 calories a day, not easy, but not to hard.
So he is something like 60, sedentary, 5 foot 10 and 280 pounds. That results in a estimated TDEE (BMR plus accounting for calorie burn of basic life activities) of 2504 calories per day.[0]
> By the time he's dropped 20 lbs his calorie counts generally need to be in the 1200s to lose weight
In this scenario that would drop his TDEE to 2395 calories per day.
At 1200 calories a day, if those numbers are correct should see at least 2 pounds a week of weight loss. If he is maintaining, check your priors.
He is either overcounting exercise, over estimating TDEE or under counting what he eats. If you have weights and accurate calorie counts you can extrapolate his TDEE from that and know exactly what deficit to eat at.