He couldn't help but grin from ear to ear when he won the prize.

grin 是动词，意思是"咧嘴笑"，通常指露出牙齿的、幅度较大的笑容，常用来表达开心或得意的情绪。from ear to ear 是固定搭配，形容笑容非常灿烂，嘴角几乎要咧到耳朵边。