ON A QUIET stretch of beach in Northern California, a father takes his son surfing. Bodies flat against their boards, they paddle away from shore. As the waves grow bigger the boy, forging ahead on his own, disappears behind a curtain of water. Then, just as the father begins to panic, the boy emerges, triumphant, riding a wave back to him.

The scene, which arrives early in Belgian director Felix van Groeningen’s English-language debut, Beautiful Boy—co-written with Luke Davies—casts a long shadow over the rest of the film....