I have a much younger cosleeping son, and he has sensory issues that prevent him from falling asleep if he isn't touching me. (As I type this on my iphone, he is draped across me asleep.) He has a double bed so that I can snuggle him until he falls asleep, then I slip away to spend time with my husband for a few hours in our room (just a few feet away) until DS wakes, at which point I rejoin DS.
Have you ever tried snuggling to sleep as an extension of sitting with him? It's such a lovely, gentle way to ease an anxious/sensory child to sleep, and I know from personal experience that the cuddles are a wonderful way to reconnect after a busy day and build the emotional side of your relationship. The warmth, pressure from contact, and rhythm of your breathing...it helps drown out other sensory stimuli. A full body pillow and heated blanket can simulate that contact after you slip away.
When I was your son's age, my dad used to wrap me up like a burrito and give me gentle head rubs until I (or he!) fell asleep, which I always loved.
Another option might be to listen to some beautiful music or do a guided relaxation exercise together in your DS' room while snuggling. Aromatherapy--maybe doing a gentle hand or foot rub with lavender scented cream, or sprinkling some lavender oil on the pillowcase, might provide a sensory cue to relax.
Another idea might be to create a "safe" sleep zone for your son in your bedroom, if you're comfortable with that, with a small separate mattress or sleeping bag--in a tent for added fun! (I can appreciate how many people might prefer to keep a master bedroom child-free, too.)
ETA: As a final note, don't discount the value of tryptophan-rich foods as a way to prime your son for sleep. A zinc and magnesium supplement can also support sleep hormones and promote relaxation.
