Manuel Oliver in “GUAC” at Woolly Mammoth Theatre (Photo by Cameron Whitman) “When you lose a son, what do you do?”   This urgent and gut-wrenching question rang in the ears of writer Manuel Oliver from the moment his son Joaquin (nicknamed “Guac”) was murdered in a 2018 mass shooting at Marjory Stoneman Douglas High School in Parkland, Fla. It’s also the opening line of Manuel’s solo performance dedicated to Joaquin, which, layer by layer, begins to provide the answer.   Fro