Get your son a colonoscopy.  Don’t quess 

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 👍

Get him diagnosed as soon as possible because if you let the inflammation do its thing and leave it untreated, the inflammation could grow to other parts of the colon, so earlier with treatment is the best option. Mine went from proctitis to pancolitis within a period of 1 month and a half

Get the colonoscopy. I waited too long for my son.  It was already pancolitis and in 3 months time, his colon was out.  I feel for you.  It is so stressful to worry about our children having this disease.  Both my husband and son have jpouches.  Been through just about everything, so I consider myself an expert of sorts although I wish I didn't have to be.  

My husband has a very successful j pouch for 13+ years now. UC runs rampant throughout his family. His niece (my son's 1st cousin) has a permanent ostomy. My hushand's sister died of colon cancer. His father had UC as well as 2 uncles and another cousin with Crohns.  My Son's 1st jpouch failed and he had a redo last October.  He's still struggling with complications.

Go to the doctor.  Don’t wait, just get him there.  It sounds like a flair.  He will be ok, but needs help now.

I hope it’s not IBD, but if it is, it appears that if biological drugs are given earlier in the disease process they are more effective.  I hope you can get him seen soon.  Many people who get treatment remain symptom free.  So stay positive either way if possible.