We either bath DD (18wo, but have done this for months) or give her a mini-massage - well, just a rub over with baby lotion, really - in her room with the lights dim. Then she has fresh clothes, goes into her grobag if its not baking hot, and gets fed the last half of her bottle in the gloom in her room before being put down. I honestly think that the whole being stripped and changed, whether its for a bath or a massage, is a cue to her. This is always done at about 7.30. And, sadly, as she has just discovered the TV, she gets a ten minute 'calm down' cuddle on someone's lap while watching The One Show. I think she believes this is part of her routine too!
I do sometimes read a story too, and I've started to do this at naptimes too. It doesn't matter whether they are interested in books, its the soft sound of your voice while getting a cuddle, that can be a cue too.
So you could take DS upstairs into a dimly lit room; make it nice and calm and don't interact with him; give him a quick massage/moisturise if there is anything his skin can take daily, or if not then maybe just a 'dry' massage, a bit of a tummy rub etc; put him in his bedtime clothes; read to him gently for a few minutes; then feed him to drowsiness?
I think they don't need a long routine, but consistency and cues are the important things. So while a story might seem silly now it could pay off in the long run. But as poppins says too, I think just doing the last five-ten minutes of their day in their room, in the dark, can make a huge difference. As long as we do this, DD sleeps the same whether she is in her own bed or at her grandparents.
Good luck!