This synesthesia is an unfortunate side effect of the mercury poisoning he was subjected to, to dim his mind and bring about his disconnected state diagnosed as autism. It is simply the nature of how it impinged on the neural tissue and the degree of effect somewhat selectively on certain brain centers that has caused an imbalance in sensing, such that incoming sensory input becomes magnified in an inappropriate way and creates inappropriate imagery in response that takes on a life of its own, and becomes an overwhelming source of stimulation that is distracting at a minimum and overwhelming all too often, causing him to withdraw and want to disconnect from the outside world for self-protection and preservation. That compounds the difficulty he has always faced with the many outreaches from family members wanting to engage with him, wanting to entice him and intrigue him and stimulate him to get him to respond in a way that could help him learn the intention behind the provocation, but all too often it has been too strong in nature and uninterpretable because of the outsized and inappropriate cascade of imagery the simplest sound will trigger within his mind, and it becomes impossible to sort things out and create an association with a linear chain of sounds, as in a spoken sentence. So he is not lacking in intellect, such that he is incapable of learning what words mean, it is that there is an extra difficulty in the translation because there will be a simultaneous eruption of visual stimuli that are highly distracting on top of every sound perceived, and it becomes an overwhelming onslaught he has to reckon with.
You are helping him overcome the trauma this has caused, and that will help him develop inner ways to cope with what is a largely irrelevant stream of non-helpful information content because it is not consistent in corresponding with particular sounds, it is quite random and so will not have any coherent meaning ever, and is thus a distraction only that is not at all helpful, but hinders. With further healing, this will fade because he will gain mastery over it in time and he will be able to reach a stage where it is just a kind of inner curiosity that he can watch out of interest if he chooses but will have no hold over him and need not impair things to the point where he will be limited because of it. That is achievable now with the other roadblocks that have been dealt with and the general opening up of the pathways needed to reach him and help to influence his thinking a little bit so he is able to align with similar goals, and not trying to obstruct the ongoing efforts to reach him and promote productive, positive interchanges he can build on through learning how to interact and communicate in increasingly effective and successful ways. You are on your way now in earnest and this will be a beautiful payoff, indeed, as it proceeds.