Grappler's Defense Against Left Hook

Hey guys, long-time grappler who's been taking my 13-year-old cousin to boxing gyms to help him learn to close the gap against good punchers.

My stand-up knowledge is limited, and my cousin keeps getting rocked with left hooks, very similar to what you see in this video (lead left hook in particular). Any advice on how to defend against that punch from a grappler's perspective?