I just wrote about this in the other thread so I'll copy and paste:

My son was a terrible sleeper from birth to age 9 years. As a baby he wouldn't sleep unless he was touching me in some way. I breastfed him for a year and he never would take a pacifier, I was his pacifier. Of course everyone assumed i was spoiling him. As he got older, it would take him longer and longer to fall asleep. His worst was around 6-7 years old (around diagnosis time). A good night was an hour for him to fall asleep, bad night 3-4 hours. He would mostly stay asleep once he was sleeping, but getting him to sleep was awful! Things that helped him: routine, CD of ocean waves (worked as a white noise), joint compressions, weighted blanket...and therapy and maturity played a role too. By the time he was 8, it took him only an hour. By the time he was 9, it took an average of 30 minutes. He turned 10 in May and it takes him 15-20 minutes tops. He doesn't need the CD or the joint compressions anymore, and uses the weighted blanket only twice a week.

Then I'll add:
I can never remember the specifics of his diagnosis either. I know he has low muscle tine and proprioception problems but can't remember the rest. My son also has anxiety along with his SPD. His worries all seem to come out at bedtime. He would cry to me, I don't know how to fall asleep!

I would try melatonin with your son. I had bought the liquid form, but my older son said it was too spicy and he wasn't willing to try the chewables after I bought those. So between therapy, doing joint compression when he's especially agitated, CD of ocean waves, a weighted blanket (also highly recommend--very calming), and sitting in his room until he was asleep--we've finally worked toward him falling asleep in 15-ish minutes. That's the long road without melatonin.

Now my younger son (not officially SPD, but he's a seeker). He was always a great sleeper. Bedtime routine and tuck him in, then he was asleep quickly. Until around Kindergarten sometime. He started taking longer and longer to fall asleep. This was around the time I bought the liquid melatonin for older son. Luckily younger one liked the taste. He takes it and then we do bedtime routine, by the time I tuck him in, it's taking effect. He's asleep in 10 minutes. He's about 50 lbs and I give him 2 chewables (1 mg each). I tried 3 but he said he was too sleepy in the morning.

If you can't do the melatonin, I'd do the weighted blanket, maybe even try joint compressions, at least on the bad nights. My younger one loved big brother's weighted blanket that I bought one for him too. He uses it every night.