I still remember when my colon was first inflamed it showed the same signs. In addition I could see puss all over stool. I was misdiagnosed in my first colonoscopy as the ulcers were barely visible to naked eye. The doctor called it just an inflamation and gave me some probiotics and gut relaxing pills to help with the constipation with a suggestion to consume a lot of bananas and milk to manage stool consistency.
I was going the same way every 1-2 hours. I felt superconstipated and had to push too hard. Probabaly not the best thing I should've done at that time.
Get the colonoscopy done ASAP, but remember colitis in its early stages may not be as visible. For the constipation part try giving your son pysillium husk with milk/water whatever suits him.
Good luck 👍