His mother often says to him, “It’s high time that you _______.”

在It is (high/about)time (that)…句型中，后面的从句要用虚拟语气，从句谓语动词用过去式，也可用“should+动词原形”的形式，should不可省略，意为“该是……的时候了”。结合选项可知，空处应用got married and settled down，表示“结婚并安定下来”，故选D。