The waiting list for getting diagnosed is so very long and it's frustrating that the GP isn't being supportive.I have a young adult daughter who is autistic and has always struggled with sleeping, so I understand what you are going through. Once she gets into bed and starts to try to sleep, her brain starts thinking about everything and any worries come to the surface. If she's over tired then she'll end up thinking everything is a catastrophe and be very emotional. When she does get to sleep, she's easily woken and it all starts again. There are nights where she's "forgotten to go to sleep", and spent the whole night sitting upright looking at the wall.
Things we found that help:
A weighted blanket was a massive game changer for helping to stay asleep and she finds the close hugging of it comforting and there's no draughts of cold air. You need to match the weight of the blanket to the child's weight. From memory I think its supposed to be around 10 percent of their weight.
Aromatherapy, lavender oil is calming and having a predictable routine culminating in using some essential oils either as a massage or just in the air can help to remind the brain it's rest time.
A quiet fiddle toy with sensory bits that they can use for calming down and relieving stress, like a tangle toy. Obviously you need to make sure it's safe to be in bed, so they don't swallow it or strangle etc.
You can get devices like the Beurer stress releazer, which you place on the chest and they have a vibrating motor in. It feels a bit like having a cat purr on your chest. It's got a light on it and it guides you to breath with it, while playing relaxing music through earbuds or on a phone. This might not be suitable for your son at the moment but maybe when older.
Listening to music, audio books or allowing them an hour of "free time" as long as they stay quietly in their bedroom.
Going to have a conversation about the things worrying them after they have been in bed for a bit, especially when they are getting more and more wound up or emotional.
A very dark room or using a good quality blindfold so there's nothing to visually stimulate them.