He has put on middle-aged spread, especially around the belly.
"He has put on middle-aged spread, especially around the belly." と表現することができます。「Middle-aged spread」は中年期に多く見られる体の変化、特に腹部周りの脂肪の蓄積を指す用語です。
似た意味の表現:
He's experiencing a middle-aged spread, with a lot of belly fat.
My father has a considerable belly, typical of the middle-aged spread.