My daughter has Crohn's but also has Gastroparesis, which is delayed gastric emptying - a motility disorder (so her stomach does not empty normally - food just sits in there for hours). Foods with fiber are harder to digest, so she was on a low fiber diet for a long time. She had a lot of soup for veggies (spinach, tomato etc) and smoothies are a good option if he likes fruit (but check with your GI first!). Considering he has a stricture, I would definitely follow the advice of your GI, even if it seems like he's eating "unhealthy" foods - for example, my daughter had to stay away from whole grain/whole wheat bread for a while and had white bread instead. She was also told to avoid nuts, seeds and popcorn and foods that were high in fat. Bland, soft foods like pasta, rice, potatoes were easier for her to digest.
Miralax does take 3 days to work. Some kids need more than just one capful - 17 g. I would definitely keep your GI updated - if he hasn't had any proper BMs in a week, they may want to put him on something stronger. My daughter used Milk of Magnesia if Miralax didn't work.
Movement is good for constipation - even going for a walk is good! Whatever your son feels up to doing.
Inflectra should reduce inflammation, but it can take a while. I think it took 4 infusions and a dose increase before my daughter felt a whole lot better. Hang in there!