Just throwing out something that happened to me, but have no idea. At around this age I started to become lactose intolerant. I would not want you to remove dairy from his diet long term because he needs the calcium but you could try and remove all foods that have dairy for a few days just for a test and see if that goes away. Mine started with mucus in the stool. Then around 11yrs it turned in to diarrhea and stomach pain. Generally mucus is from irritation. Keep a food diary of everything he is eating so you can track it and see if it is something else he is eating. If you keep a diary long enough (include even if he eats a few chips, everything in the diary) you should be able to identify what he eats and narrow it down. I just said dairy because that's what happened to me. But can't say if that is happening to him. If you have another family member that is lactose intolerant than that becomes more likely.
Thanks for the info. I'm going to try what you said and restrict his dairy intake for a few days. I don't have any knowledge about lactose intolerance, but I'll read up on it now just to be informed. Interesting that you mentioned other family members since he has 2 cousins who have quite a few food allergies, one being a celiac.