My nephew was diagnosed at about 13.
Unlike your DGS he did have a bowel resection without a colostomy aged about 17? He is now 32, works as an engineer, is married with a little boy and fit and well.
He too avoided eating certain foodstuffs (I remember him having a fairly restrictive diet) and has had odd bouts of recurrence generally through eating something not on the list, other than that he's 6' tall and doing well.
It was very much trial and error 20 years ago, so it sounds like things have greatly moved forward in that time.
I truly hope everything will go well, best wishes to you and your family x