I would be thinking you would need to expose him to much more than a walk around the neighborhood. Try and get him used to things you may see or come into contact in your therapy sessions. If this was me , I would start by taking him to a local horse show or somewhere he is exposed to people and crowds. Crowded spaces, people approaching and patting your horse at different angles, wheelchairs etc. Take him out on as many day trips in the horse float as possible.
Id even look into things like exposing him to different surfaces. As you take him to different places, there will be heaps of different surfaces like, tiled floors, pavement, etc.
If I was purchasing a horse for this reason , I would be looking at a horse that is well behaved in any situation and is calm at all times no matter what is happening around them.
I dont mean for any of this to come across as being harsh, I just think there is a lot more too it , in training a therapy horse.