Hello. I would like to say I sympathize with you. My daughter was diagnosed with SPD at three and is now 6 years old. I have struggled with sleep with her pretty much since she was born and I look forward to a life of sleep issues :)
For a long period of time I took responsibility for her sleep. Finally about a year ago I realized that this is not my problem as I can't force her to sleep. I chose to give her power. The power to sleep or stay away. There were a few rules. 8:00 is adult time. You don't have to sleep but you have to be in your room, lights off. I purchased a book like and a night light. If your child is like mine, it takes longer than average to get them to acclimate to a new routine. However after about a week, I didn't hear, but it's not dark, I'm not tired. It was simply in bed, lights out, end of story. Sleep was optional. Low and behold she was falling asleep with in a 1/2 hour.
Same rule applies for dark. My daughter has an occasional 4:00 am wake up. Rule is grab a book, stay in your room. Sleep is optional. Giving her this power has removed all struggles. My telling her she had to sleep was a huge issue for her because sometimes she doesn't want to. However, Mom needs her day to end. This cures all.
Finally, SPD kids are very, very sensitive to their surroundings. My daughter has anxiety over things we do not consider to be an issue. So, when she is "working something out" as we like to say, her sleep pattern is often disrupted because she cannot turn her brain off. Watch for what is going on in your son's life, it can be as simple as an overnight trip or rearranging furniture in the living room. See if his wake ups coincide with these types of things.
I understand your frustration and I promise you have a lot to learn. However as you learn it will get easier. Feel free to contact me off line if you have any other questions. I'm happy to help.
J..