If he is in the "little or no exercise" category, he should aim to get into "Exercise" category. Exercise (in reasonable amount) is healthy whether you loose weight or not.
He should not be aiming for "basal metabolic rate".
For an animal model I imagine it means being fed less than would have been eaten ad libidum, or less than was eaten by a pair fed individual.
But considering you are probably in like the <= 3rd percentile of healthy humans in terms of BMI, your 'normal' calories are pretty damn low and probably commensurate with what would be considered calorie restricted.
So if I'm eating more and gaining lean muscle, I could still fall under the calorie-restricted category?
Basically I don't enjoy being thin but I do want to live to 120. I already do calisthenics and basketball regularly, just wondering what else I could do to min-max my longevity.
I don't think mechanism is worked out well enough to say for sure that being calorie restricted for someone like you that already is lean and not at risk of obesity related disease would be beneficial.
My first guess would be that even if there are longevity benefits for calorie restriction, depending what the mechanism is, you could either be already enjoying them by virtue of clearly not eating that much relative to your lifestyle and metabolism, or alternatively that any benefits of starving yourself enough to clamp down on mToR or some other hypothesised mechanism may well be counterproductive by pushing you close to malnutrition or just increasing your risk of injury or whatever.
I guess you could do a self experiment if you were really interested, try a fasting mimicking diet regimen where its five days a month every month to get the benefits of calorie restriction, and see if you can maintain lean mass while getting improvement in markers.
