My cousin had similar symptoms. Have them check for Colitis. It can cause blood in stools. Usually means a special diet of Gluten Free. If it is this it is very simple to overcome. I would recommend using holistic meds (they are much better than the stuff the regular physicians will give) and a special diet. Remember to educate ALL those who are around him so they do not give him something he cannot have too. This was a problem with coaches, friends, and certain relatives. Understanding that if he has Colitis it can cause hospitalization if you don't stick to a stringent diet.
Chron's is another possibility. Along with Diverticulitis/Diverticulosis. Although he's very young for the latter.
Both Colitis and Chron's can be diagnosed using a Pill Camera. Basically it's a pill looking thing that he'd swallow...it takes hundreds of thousands of photos and passes through like regular food would. The docs can upload these photos and get a very good idea of what they are dealing with.
Good luck and just make sure you keep him on a bland diet right now. White rice, bananas, jello...things like that. Clear fluids. Steer away from too much sugar. Don't wait 6 weeks. Please. If it is Colitis or Chron's then 6 weeks is far too long! Find out if you can get an Emergent Appointment. If need be take him to the ER and make them take care of this. Blood in the stool is an Emergent Situation. He needs to have been seen the day this started. Don't put it off any longer.
I am not a Doctor, just a mom like you .... there can be many reasons. Keep calling the GI's office and ask if anybody has cancelled. Sometimes the more you call they will get you in to get rid of you on the phone LOL!
In the meantime, if ok with your Ped, maybe try eliminating milk and see if it helps at all .. also, an anal fissure (cut) can cause this from constipation .. but I'm not a Doc .. just some thoughts for you to run by your Doctor.
Cheryl