Are there suggestions we can make to him at the end of our sessions to speed his progress?

Simply applying your intelligence and awareness from what you are learning through the communications from him will be of value because you are getting pertinent information and it is all part of the puzzle we are helping to unravel for his and your benefit as well. So as you think about this now and the recent exchanges and all he has to contend with as his mind explodes with imagery with the least stimulation, you can coach him about those visual impressions being a kind of aftereffect and byproduct of simply being stimulated, and that they have no higher purpose or meaning in and of themselves, and even though they are dramatic and he is used to being preoccupied with them, it is not useful as an exercise and will only divert him from paying attention to what is truly important, that will really help him understand the outside world and those trying to communicate with him. So you can encourage him to let go of those images, let them pass on, and focus on the sounds themselves spoken to him and endeavor to look at the body language of people talking to him, including their facial gestures and what they do with their hands as a clue to the meaning and purpose the sounds are trying to convey. It is very much like what a mother does intuitively with her baby, to move in close when she wants to talk with it so she fills the baby’s visual field and is close up and not across the room so the baby can study her face and see her smile and associate that with the good feelings it is picking up, as well as the soothing sounds the mother might be making and the musical joy of hearing her sing a lullaby or the rhythm in pacing and tonalities of a nursery rhyme that is repeated and becomes familiar and associated with "pleasant time with mom" and will grow in value in the experiencing with each repetition. So he has missed out on much of that joy of life and the interplay with his loving family. It is not too late to reexperience those kinds of encounters now and this could be a suggestion for the family as well, to adopt the patience they would have with an infant and make something happy and pleasurable when they have an interchange with him. This will encourage him and gain his attention and give him an incentive and a reward as well to maintain a more long-lived and intense focus on them, and that will be instrumental in helping him learn what the sounds of language mean. There will need to be lots of repetition to build in that knowledge finally, at long last, but the payoff will be worth the effort.