Sorry for the late response, but definitely check out the parents' group - many of us have older teenagers/young adults (but they're still kids in our eyes!!). I think checking his trough levels of Humira is a great idea. He may need weekly shots. Some kids, especially if they are really inflamed, require higher doses of biologics - they tend to metabolize drugs faster than older adults.
When he had scopes, do you know where the inflammation was located? Was it in his small bowel or colon or both? If in his colon, was there inflammation low down, in his rectum or sigmoid colon? Because sometimes local therapies, such as enemas or suppositories can really help with bleeding, frequency and urgency. The bleeding could also be from a fissure or hemorrhoid. Does he have pain with BMs?
It is a good sign though that he is only bleeding a little and that it has improved somewhat. It does sound like he is at least responding to Humira somewhat.
The best advice I got from parents on here when my daughter was diagnosed was to think of this as a marathon, not a sprint. Things move slowly in GI land. Inflammation that causes ulcers and bleeding does not happen overnight and it can take a while for healing to occur. It also sometimes just takes time to find the right medication combination that works for your child. For some kids that might be Humira every other week, while others might need Humira weekly, and still others might need Humira weekly and an immunosuppressant like Methotrexate plus suppositories or Proctofoam. It really is very individual.
Hang in there, keep us updated if you can and good luck!!