Why the belly bulge?

57 Male Client with 13% body fat (athletic). Why does his stomach bulge outwards? We work abs, he does cardio, eats really clean and healthy, yet his stomach sticks out past his ribs. If he makes an abdominal muscle it seems to be all muscle and toned, yet he says its way larger than when he was younger. I am stuck on what to do. I have considered a dairy allergy, conflict with his medication, etc. but it's outside of my scope of practice at that point... I doubt there is a magical exercise to help with this, but he is looking to me for answers and so now I look to YOU!!! Thanks!