First question to ask yourself - what are the things he needs help with that a dog can help with (that are not companionship)? Get this list together (and feel free to post it here if you want to).
Next, for each of those, what can the dog do?
For meltdowns common things that dogs do are providing deep pressure and alerting to or attempting to prevent self injury. They also will sometimes redirect the kid to doing something explicitly with them - I've heard of dogs bringing their brush to their person and making the person brush them.