Would using sign language be a more productive way to communicate with my client to bypass his synesthesia problem with sound?

This is a viable approach in certain respects, not necessarily learning the formal language equivalent and trying to train him. Because of his high energy within his mind, keeping him focused on something is more difficult unless he has a keen interest. Whether he could be engaged with and enticed to begin to learn in making something like gestures a kind of game is worth trying, but could not likely be taught in the usual way because of his ability to focus and willingness to pay attention and react in a desired way. He has learned to be independent. It is usual and expected that he will control his self and his actions within the environment. He has been trained to accept certain limitations on his behavior and movements, but that is not the same thing as choosing to be independent and ignore what is going on around him when he chooses. So the challenge will be to engage with him in a way that can have a chance to work and have him devote the requisite focus and attention to begin making the needed associations and learn what is intended, so that a gesture could be an effective language, in effect. So this is something worth the attempt by his family members. If this is given some thought in advance to think about what would be most desirable to convey to him in the way of a request for a response to accomplish something useful and practical, that would provide an extra motivation, not in him just responding to another person because they want something to happen, but seeing that person communicating in a way to show him something he can gain a benefit from. And so we leave that to the ingenuity of his family to think of things they may have tried but abandoned in the past and think about reintroducing something along those lines in a fresh setting now that he is older, to see what inroads they can make to engage with him and help him make a connection to gain a true understanding that he can have a meaningful exchange in this way with other people.