It sounds like he's trying to play with your son. I'm guessing that Onyx is very young?
Keep him on a leash and don't give him access to your son's fingers/toes. Work on teaching a settle command by rewarding him when he is being calm and quiet. Stepping on his leash helps keep him in one place. At first just place a high value treat on the floor so that he doesn't move. Once he lays down calmly for the treat, start saying in a calm, soothing, and low key voice "settle".
I think they usually pick up on it pretty quickly. Once he has mastered "settle" work on teaching him "place" or "mat". This gives him a location to go to and settle.
He'll get it and it will take a little time. I don't have any experience with Cerebral Palsy and dog training so I can help you with the right now portion, but there are some experienced trainers on this board that can hopefully help you more with that.
Just curious, but are you hoping to use Onyx as a service dog?
Welcome to the board!