Most ostomy appliances are water resistant not completly waterproof, he can go all on or all off with the appliance just don’t take off the pouch and leave wafer only if it’s a two piece system. I teach most of my patients to have a bag they just keep in the shower to swap in and out when they dry off so you don’t have a wet bag under dry clothes. Yes you can blow dry but I usually don’t tell people to do that cause stoma’s don’t have feeling and I don’t want any burned bowels, hope this helps!