Son is 17 and recently began having toilet problems

Thanks for that asdballerina26. We hope he'll get over it too when his reason starts to overcome what he calls his "crazy thoughts".

Don't worry, it happened to me too when I was 2 or 3 as I have learning difficulties. I was in diapers till I was 4 or 5 and ended up in hospital twice as I refused to poop. It somewhat still affects me as an adult though and I can't understand why. 

Well, I was kind of hinting in that direction without wanting to explicitly suggest you look there. The GP can provide far more value in identifying appropriate next steps.

Some people are gluten intolerant so that's one possibility.

https://celiac.org/about-the-foundation/featured-news/2013/06/gluten-sensitivity-linked-to-autism/

Another possibility is he has sensitivity to FODMAPs. Some vegetables are high in FODMAPs. For example broccoli and onions.

https://www.health.harvard.edu/diet-and-weight-loss/a-new-diet-to-manage-irritable-bowel-syndrome

It's not easy to figure out and each person is individual. Something I would suggest is keeping a food diary where you write down what he eats and then also write if there seem to be more toilet problems after that meal, or the same day.

Not sure that is helpful but I hope so!

Breakfast for my son is a bagel with choc spread and a piece of fruit and a drink of juice.We have various lunches usually with salad, and a hot evening meal usually vegetarian and something healthy. He also has a glass of water with lunch and evening meal. 

What does he eat in a typical day, if that isn't too nosy a question?

I ask because diet is one of my special interests. Some foods affect people in different ways so it's possible there are one or two foods he's eating that are causing constipation. Another possibility is something he's eating is making having a poo painful or uncomfortable as it could be something that doesn't agree with his digestive system.

Thanks Just Some Guy what you say is food for thought. He says it doesn't hurt him to go now but he's scared and my wife notices he likes to smell his faeces. By the  way. By trained professional support do you mean cahms?

This feels like cause for a conversation with a doctor. There may be an underlying physical issue causing him pain or discomfort that he's not willing to discuss, or it may have been a one-off thing with lingering mental impacts. If it's the former then medical support will be sensible, if it's the latter then your GP will be able to suggest whether things like stool softeners or a change in diet might assist, or trained professional support will be needed.

Thanks for your honesty Robert123

I assure you that your post is not been ignored. Just, I, like many people here have no practical or useful advice, thus the silence.