I post a lot on the sleep boards. If you don't have a problem with your DS sleeping on you acorn, then let him sleep on you, it's fine. He will associate going to sleep with being held or on you, of course he will. But that in itself is not a problem unless you don't want him to sleep on you.
My best friend co-sleep with her DD for nearly 6 years. She was not ready to self settle before then and my friend wasn't prepared to remove her DDs comfort so they co-slept quite happily. I had my DD around the same time, but I did CC at 12 months because I needed her to not need me to sleep.
(incidentally, when myself and my best friend went on to have other children, we made sure that Mummy was never established as their main source of sleep comfort from early on)
Brain studies show that babies/children cannot just lie down and go to sleep like an adult does until school age, so aged 5 or 6. They need something to help relax them - Mummy or Daddy present, feeding, cuddles, dummy, "blankie", thumb suck - whatever it is they use as a comfort to sooth and settle to sleep.
There is absolutely nothing wrong with Mummy being the comfort sleep trigger for a child. In fact it is normal and natural. Stress and problems only arise when it is decided that Mummy can no longer be the sleep comfort, if done before baby is emotionally ready.