My daughter has also been on Miralax for many years due to a motility disorder and constipation with her Crohn’s. In the past, she had more constipation and was on two capfuls of Miralax daily when her Crohn’s flared. Now as she’s grown up, she tends to have diarrhea more when she flares.
Sitting on the toilet every day for 10-15 minutes every day can help retrain the bowel unless it’s inflammation causing the constipation. It’s not a bad habit (in fact, it’s good to make it a habit) but if it’s inflammation causing the constipation, it’s really not going to do a whole lot.
Adding fiber caused a lot of cramping when she had active Crohn’s. So I would be careful with that. Making sure he’s drinking enough water is important though (especially if you add fiber).
I definitely agree that shakes like Peptamen Jr or Pediasure peptide are a good idea to help him gain weight. My daughter was severely underweight due to Crohn’s and the only thing that helped her gain weight was these supplemental shakes. Shakes like Pediasure peptide and Peptamen Jr are semi-elemental (partially broken down) so much easier to digest than Ensure or Boost which are polymeric shakes.
I also want to add that being malnourished can also cause nausea which then makes the child want to eat even less and then you get into this vicious cycle where they’re nauseous, can’t eat, become malnourished and then become even more nauseous.
His Humira dose is on the low end. My daughter was on the adult dose of Humira before she was 88 lbs. in fact we even switched her from 40 mg biweekly to 40 mg weekly before she was 88 lbs. She was 13 at the time. She was probably between 80-85 lbs at the time.