Direct pressure to the belly button with the palm of the hand (not quite enough to be uncomfortable, but almost) sometimes has a remarkable calming effect, particularly on small children.
Borrowing from Tibetan grieving practices, actively commemorating the uncle's death at 7:15pm and doing something calming to help his spirit move on can make the reliving of the experience (which is going to happen anyway) into something deliberate, which can evolve into something voluntary, which can evolve into something optional. The Tibetan standard prescription is every day for a week, then every week for a month, then every month for a year, and then every year (on the anniversary) until convinced that the spirit has moved on. A persistently trapped spirit like the uncle's might need more help than he has had so far before he stops haunting people.
Having the right attitude to nightmares is also important. I believe firmly (and will post separately in detail about this) that nightmares are an attempt by the brain to forget. It's actually good that he's having the nightmares, because he needs them in order to forget; the challenge is to make the nightmares bearable enough that he is able to fall back asleep as quickly as possible once woken by a nightmare. Comforting weights are definitely a good way to do this; in my view the most comforting weight possible is a parent's arm.