Adults with disabilities certainly can be taught to use the toilet appropriately, and the sort of teaching you would use with a child is often still effective. A common method of doing this involves blocking out a few hours, giving him lots to drink, and bringing him to the toilet to give him the opportunity to practice. If the issue is "aim" rather than him having accidents, a way to address that is to arrange differential reinforcement for accuracy on his part. Having a visual stimulus prompt (as you suggest) along with instruction and reinforcement (i.e., praise) when he hits the right spot all sound like good ideas in this case.
Many thanks for your response and tips. It is very much a 'hold and aim' issue with him, rather than him just having 'accidents'. Very much appreciated advice and I'll make sure it's also passed onto his family.