Hi!
I used B Braun two pieces system and I showered everyday without any problem. I used to do it in the morning before breakfast, so the bag would have been almost empty and the whole procedure easier.
I emptied the bag from the night, then I used to shower normally. After that, I removed the wet bag, I dried carefully the plaque (I always used the double adhesive plaque, with protective paste all around the stoma) and then I applied the new bag.
I used to change the plaque every 2-3 days (or when the skin around the stoma became itchy below the plaque...) and the bag every day, as my stomatherapys stuggested me.
Honestly, I would not raccommend blow dryer because the hot air may dry the delicate skin around the adhesive and that would be really awful.
Ps: the adhesive of my system NEVER came unstuck! Neither after showers, neither after entire days at the beach. I am sure that you and your son will find your own way to manage all this, I send you my humble suggestions and experience and a big hug.