Hi,
My DD (just 7) is going through something similar, she is very anxious, is having trouble sleeping and is having 'bad thoughts' about things happening. I have not been well for over a year and had to be taken to hospital in an ambulance last month which has stepped up the fear.
What has helped is:
-Giving her a comfort object again, in her case its a puffle and a Team GB mascot that she loves and takes around with her. This worked during the day but at night she needed more reassurance. I spend time with her now teaching and practising her very simple relaxation exercises (breathing and stretching). Also I told her about a place I go to when I am trying to sleep, a valley with a castle and a lake in the middle. I asked her to meet me there whilst we were asleep and we spent a long time talking about this and visualising this - where we would meet, what we would see etc. In addition I gave her a cheap moonstone ring that she could wear that I told her my mother gave me (true) when I was having nightmares as a child. I told her it banishes bad thoughts. It lives in a box in her room for when she really cannot sleep. She can also turn her light on in the night if she cannot sleep and read for a little while, she was lying there for hours not sleeping and knowing she won't be shouted at for having the light on has made her relax a bit more. Under her bed is a special box with lots of happy photos and memories in it for her to look at if she needs it.
-I gave her a little notebook and a pen for her bedside table and she writes all her fears in there and we discuss them the next morning. It also has a list of all the the things that make her happy, big and small. Also it has a little rationalisation that nothing bad has happened to her in 7 years, nothing bad has happened to me in 30 years, nothing bad has happened to DH in 34 years, nothing bad has happened to my mum in 62 years and nothing bad happened to my gran in 95 years (there have but nothing DD knows about).
-We keep her as busy as possible, tiring her out physically. For example today she went to the park for several hours, then I got the paddling pool out. SHe was much better as she was so tired she went to sleep very fast with little time for brooding. She doesn't get much time to herself to brood, I try and fill her time in every way I can.
-Generally I give her lots of cuddles and comfort, plenty of praise and reassurance and reminders that its my job to keep her safe and that she doesn't have to worry about these things. We have also been pushing boundaries slightly to prove nothing bad will happen e.g. today for the first time I walked her to the shop, gave her some money and walked home. She bought the sweets and ran back home (about ten metres tbf). She was really frightened last night that someone would steal the money/attack her/try to steal her/run her over, but after several hours of confident reassurance and building excitement she did it. Then she got to eat her sweets and watch us eat ours that she had chosen for us. Now we have an example to use next time she gets frightened.
The most important thing though is that she knows she can talk to me any time about the silliest of fears and I will not laugh and I will always take her fears seriously. Then I gently try and reassure/rationalise etc with plenty of cuddles and comfort.
DD has been so frightened and upset which has thrown me as in all other ways she is hugely physically and socially confident but she has responded so well to these tactics. I really hope something here works for your DS xx