We struggled with this, too. Thigns that helped a ton:
(1) No milk after suppertime. (Seemed to make them konk out too much to get up.)
(2) Take him to the bathroom 1 hr before bed and again at bedtime...he can sit until he goes!
(3) Stay strong with the laundry! Make your son help pull off the potty bedding and carry it to the washer. Although it sounds yucky, if he has to help, he might be encouraged to try harder.
Note: If he takes any allergy meds or any other required day-to-day meds, you might check to see if a side effect is sleepiness. If so, it might be making it harder to get up when/if he needs to go during the night. Some kids truly have smaller bladders (my 3rd child did...so it just took her longer to grow and be able to hold it all night...or wake up on her own).
My final step sometimes feels worse than washing the bedding...it's setting an alarm for yourself to get up and wake him and take him. However, my children's bedtime was 8pm, while we head to bed around 10/11pm. That tended to also work quite well as the "middle of the night" potty trip. As well, when I got up to prep my hubby's things for his day early - I would wake my daughter...and then put her back to bed (it was still dark out!). She'd continue to sleep more ... but I'd save myself some laundry!
I think what helped her the most was having to help with the laundry. We used the vinyl mattress cover from Target (elastic edges & waterproof). While I wiped one side of the mattress cover with a Lysol or Clorox wipe, I had her doing the other side. She didn't really like doing it, but since it was a team effort, she came to understand cleaning up from these accidents required much time and effort & though I loved her dearly Mommy needed her cooperation. It worked wonders!
Good luck & Lord bless you!