Heavy how 2 tell asd son his dog needs 2 be put down

There's a good deal going on here. Autistic individuals feel and sense-perceive with far greater intensity. We cannot filter out information like non-autistics, so the world is Too Real. What we need are practical solutions, learned virtues or principles (as we won't learn them subconsciously or telepathically), and literal step-by-step instructions or guidelines for nearly everything in life from the emotional/psychological to physical and matters of the soul. 

It sounds like you've tried to shift the conversation to emphasise the cruelty of allowing an animal to live in pain. But I think the problem here is that perhaps your son is in an state of feeling so overwhelmed and a kind of intense emotional pain, that maybe he feels the dog is the only one he can Relate With (or share a sense of Empathy). If he loses the dog, the isolation of being then becomes unbearable. 

It could be good to find an Autistic Therapist. Feeling understood for an Autistic means Being understood. Most of us have a really difficult time with language, so the power of using language for hypnosis doesn't work on us. The use of language to change how we feel doesn't work. We need practical, actual solutions. We can sense when someone is responding in a way that feels different to what we're trying to express and many times we might not have a good command of words until our mid 30's or so. Different forms of expression can help such as arts and sport (usually things which are self-building not competitive or group oriented). What outlets does your son have? 

During various transition stages, I've not only let my son binge on video games, but reinforced it. It can curtail the incoming experience, slow down the gigantic wave crashing in and allow room to breathe by simply shifting focus. Every process takes so much time to work through and transitions create an enormity of emotions. For an Autistic, since we cannot filter out the things we sense-perceive very well, big emotions aren't just big. They are All Encompassing. 

The second thing that really makes it difficult for us is a drive to Resolution. It's part of the monotropic brain, hyper-focused and analytical. https://monotropism.org And with language difficulty this is also a compounded, overwhelming experience. We need fundamental understanding of things to be able to finally cope - the physics and biology of things, the deeper understanding of how all the parts work together. 

And if you can, have a chat with the Vet to see if your son might be allowed in the room while you put the dog to sleep. He might feel overwhelmed with sadness and that would be a good time to allow him to cry through the process and just be next to him - encouraging him to cry it out. It's remarkably sad, but does provide a type of closure, especially if he feels a part of the process. 

A support animal will unfortunately have a shorter lifespan than we do. If there's a possibly of maybe getting a kitten, something new for him to care for, it won't feel like 'replacing' his dog. But this kind of life-stuff is important for anyone to go through, unfortunately. But I might suggest, if you can and you feel this could help, see how he responds to an exchange of upgrading his video gaming for taking responsibility for the humane act of releasing his dog. Doctor Who is a good one for helping us understand these kind of humane acts as well, if you enjoy watching that together.

Thank you, that post helped me integrate some more of my life experience...

Thank you, that post helped me integrate some more of my life experience...