Welcome to Healing Well. I'm glad you found us!
I had the exact same problem in the beginning, and here's what I did to fix it. I ordered wafers that are a size larger and cut the hole off to one side. I moved the stoma guide up and to the left a little when marking it, so that it moves the wafer a few centimeters away from my navel.
Once I put it on, I lay down and hold my hands over edges to warm them up and smooth them out. This can take a minute or two. Then, I wait to put on any clothes that are tight, like jeans, until I know I have a good seal.
The other thing that helps is to shave the hairs off the belly around the stoma and make sure the skin is completely dry.
Also, the paste is not an adhesive. I don't know why they call it Stomahesive! That just confuses us. Just put a small amount of it around the hole that you cut in the wafer. It will keep the output from getting onto the skin, which will prevent leaks.
I've had excellent results, 7+ days of wear time, with the Coloplast Assura Extended Wear wafer (item #2833) with matching pouch (item # 13986). I highly recommend them! The wafer has a built-in skin barrier, so it is meant to go onto dry skin.
I hope your son is doing well. The good news is that he is UC free for life!
Dx'd Crohn's in '99 at 28. Proctocolectomy and ileostomy in '06.
Disease-free, medicine-free, and very thankful to be healthy again :)