My daughter has never been off meds. But she certainly was not the most compliant patient when she was in her early 20s! It depends on the kiddo and the disease severity, I would think, but my daughter's fecal calprotectin has been under 50 when she's been in remission. It's usually in the 20s but once or twice it's been <15!!! She is currently on Remicade and doing well.
My daughter has a friend who was diagnosed when she was 19 or so and refused biologics for years - she was on 6MP instead, despite a GI warning her that she would need surgery if she didn't go onto biologics. At some point when she was around 23 or 24, she stopped 6MP to try various diets and natural remedies. Both my daughters begged her not to stop meds and to consider biologics, but she was adamant she didn't need them.
Now, she's 28 and has narrowing in 3 parts of her small bowel and will likely need surgery. She's now on biologics but scar tissue has formed in at least 1 of the areas and possibly in the other two.
She's very afraid of surgery and Crohn's returning where she has surgery and causing further strictures and really regrets stopping her meds and not going on biologics much earlier.
If I'm remembering correctly, your son had pretty severe Crohn's in the past and had surgery, right? I would highly recommend that he speak to his GI about the risks and benefits of stopping biologics, considering his FCP has already started going up.
Personally, I think his FCP shows clearly that Remicade was keeping him in remission and he really should either get back on it or try another biologic.