I am nt (I think) but have first hand experience of a dog helping with anxiety. Its just my personal experience, but I'll tell you my story to see if it helps.
When I was a teenager (about 18, so older than your ds) I developed severe anxiety and agoraphobia. I had been ill for a few months when my mil (then my bf's mum obviously) found a german shepherd in the local park. She was in a terrible state, riddled with fleas, terrible skin infections and sores round her neck where she had rope that had rubbed at her skin. She had been seen tied to a tree by several people, but then disappeared. When mil found her she was collapsed under a bush and close to death.
At the time the only way I could get out of my parents' house was to go directly to my bf's house by car and straight in the front door without stopping once I arrived. I happened to be there when they found the dog.
I instantly fell for this poor exhausted wreck of a dog and took it upon myself to nurse her back to health. I bathed her daily, treated her flea and skin conditions and dealt with the awful continuous diarrhoea caused by starvation and parasite overgrowth. Then a strange thing started to happen to me. I realised that worrying about and taking care of her, had taken the focus away from me and I wasn't feeling as anxious. Yes I was worried about her - but in a healthy way, iyswim.
After a while I found I was able to go with her to the local vets to discuss her care and treatment and ended up doing this a couple of times a week at least - whereas before I literally never left the house, except for a short car trip to my bf's house. Then when she was well enough to start going out for walks, I was able to very slowly build up my ability to go outside with her. We started off just walking to the corner - she was weak and I was scared, but we did it together. Eventually I found I could walk her in the park daily and people would come up and ask how she was etc, so I started getting more contact with other people.
We went from strength to strength together and eventually I was able to ease myself back into normal life, with her constantly by my side. Finally being able to go to work part time and eventually moving in with my bf (now dh) when he went to uni and get a good job that supported all three of us.
She was the most calm, beautiful soul I have ever encountered in a dog, loved by everyone she met, despite being the biggest, longest coated shepherd bitch I have ever seen. The vet reckoned she was about 7 when we rescued her and she was with us for another 7 1/2 years, so in other words, lived till at least 14.
I really do credit that amazing dog with saving me and it was through her that I developed a lifelong love of dogs the lead me to take up heelwork to music, agility and obedience and also to do some study and obtain some canine related qualifications.
I'm not suggesting you should get your ds a dog that needs nursing like mine did. I just wanted to explain how having another creature's needs to focus on was what helped me to overcome my own anxieties and move on in my life.
Personally I think that getting children involved in the care of pets teaches them such a lot about caring for others and responsibility. Its an important life lesson for them that we have to pick up their mess every single time they do it, that they need feeding on time even if your favourite programme is on and that they have to be walked even if its cold and wet and you'd rather stay in and play on your computer etc. I think if handled right, pets can be so valuable to families/children be they SNs or NT.