I take the Wal-Mart Equate liquid brand of Anti-D/Immodium. When my bowels are in spastic mode (where there is constant pressure and I'm going every 1-2 hours) I'll take a teaspoon or two of this and that usually clogs me up for the next 12 hours. If I go again within a couple of hours I'll take another tsp or two.
My heart aches for your son having this illness at 14-years old. Bless his heart. I hope the Imodium alone will get him through his Boy Scout trip and that he has a GREAT time.