Mine leaves it out when just in a t-shirt in summer, often overhanging his gut, or worse, not covering his gut as the t is too short & doesn't go down to his waist level!
Tucks in if wearing as a vest under a casual shirt.
Casual shirts an issue too as 'tucked in' emphasises gut; 'tucked out' leaves the overhang problem.
Any tips? He's trying to lose weight but a looooong way to go!