You are doing a fantastic job in helping your nephew, and the replies you've had have been wonderful! It must be very frustrating for your nephew not finding good stuff when you are continually doing so. I dont know his type of head injury or how serious it is, but it is possible that he gets confused by the signals of the detector. You could try seeding a plot that is easy to dig, e.g. sand lots, and seed it with coins and some junk items, and see which ones he finds. If he is still going for the junk, then it would point to a difficulty in interpreting the signals and in that case, the best and most expensive detector in the world wont do much good. Multi toned detectors may also be confusing to him, but he may not be able to realise it himself or explain it to you. A little subtle testing will help you to understand what is happening. If the tones are confusing to him, then a single toned detector where you can set disc. to knock out signals from the rejected targets (e.g. Tesoro) might be the way to go. I wish both you and your nephew well. It would be great to see the hobby detecting industry supporting people like your nephew, who have given so much of themselves.