Prednisone worked but Calpro still so high

My son (now 18, diagnosed with severe pancolitis at age 12) experienced a flare this winter after about 6 years maintaining remission with 4 tablets Lialda and Rowasa (a few times per week). For past 6 years we monitored calprotectin levels closely and whenever they started to rise above 250 he would begin to feel more stomach discomfort and gassy and had a few episodes of bleeding and loose stool but he was always able to get it pretty rapidly under control with nightly enemas (one time did steroid enemas instead). The cal pro would quickly come back down and he would feel better.
This winter the numbers started to climb and he was inconsistent/resistant about the enemas. By the time the numbers got up to 1100 he was in full flare. He is just now finishing a 6 week course of prednisone (1st time on prednisone in 6 years), which worked really well, he says he feels great...but he just got lab work back showing his cal pro is still at 1100. We don't know what to do now. Seems like with numbers like that, despite feeling well, he will soon flare now that he is off prednisone. Any ideas why the prednisone would be great at the symptoms but not touch the calprotectin number? Not sure if we should just try to continue on a low dose prednisone for a few more weeks, immediately start biologics in anticipation of relapse (dr would like to go with Stelera or maybe Entiviyo), or perhaps just wait it out and try night Rowasa for a month?
Honestly the main thing that is different now, compared to the first 6 years, is that last year he 1) went off his gluten free diet --and I don't think he is open to going back bc he was so skinny on gf and his doctor tells him the gluten is not the problem, and 2) he used to take high doses of CBD daily but now doesn't.
I've been trying hard to back off nagging him and letting him experiment with taking control of his diet/meds, but its hard to watch him backslide.
Advice?

Personally, I would encourage him to go back to the enemas & see where the numbers are in a month. Myself, I would not do more prednisone even at a low dose. If it ends up moving to a biologic, my pick would be Entyvio, it targets the colon & the least systemic.

It's hard being a teen & even harder being the parent.

It may be time for a biologic like Remicade or Entyvio in addition to the Rowasa enemas. I was deadly afraid of these biologic medicines. But when you look at the research, they are extremely safe. I was on Humira for five years and never had a single side-effect. In addition, the biologics can change the course of the disease, which is crucial for someone who was diagnosed with UC at 12 like your son. My son also has UC. He was diagnosed at 4 years old! He started taking Remicade at 9: full remission and no side-effects for the last two years.

Wish you the best with your son. Don't hesitate to ask questions.

Thank you all so much for your input. I could not have gotten through the 7 years and kept him so healthy without the help of this group. For those who do switch to biologics were you able to stop doing enemas?

When I was on Humira I was able to stop the enemas for about two years. Now I have just recently started a new medicine (Rinvoq) and am in the process of getting in remission. I am currently using both the enema and suppositories until everything is under control. Then I'll try to put away the rectal meds. But if I were to feel any inflammation starting, I would start the rectal meds again. I think a lot of people in this forum follow a similar protocol, which may also help your son.