This may not at all work for you but...I co-slept with my twins until they were 5 yrs old, (they're now 8)at which point they transitioned without any problems into their own beds into their own rooms and sleep alone fine now.
When I was PG, I'd read all about Attachment Parenting and The Continuum Concept and realised that it's unnatural for young children to be expected to sleep alone. Their natural instincts are to look for their parents/ carers, especially at night in the dark.
That's why they say it takes so long - for SOME children, not all - to comply with the 'return to your own bed and go to sleep thing' - because it goes against their instincts and why it's also hard for parents to enforce it - leading to inconsistency. We're sort of designed to snuggle up in a heap all together!
I realise this may be a complete anathema to you and many other reading this but I'm only saying it to out your child's behaviour into a context where it's only doing what it's pre-programmed to do. If you can understand better that this is it's motivation, then maybe you will respond differently, even if you still plan to 'train it' to sleep alone.
As I'm single and had children late on in life and never expected to have 'an evening' for several years, once I'd had children, then maybe it was easier for me to do this. It's also meant that when we co-slept, I went to sleep at the same time as they did. This might not at all work for you.
A compromise - although this might confuse your child at first, as you're already trying the sleep-alone-training thing, is to lie down and snuggle for maybe half an hr with your child, until they drop off to sleep. That might make it feels secure and happy enough to then relax and sleep. At this age, children need loads and loads of physical contact and we don't always have time during the day to give this. I was still b/feeding my twins at that age (till 28 months) and most of their pleasure was because they could feel/ smell and see me close up.
Now the rider to this is that, if I had my time again, I might have done a lighter version of sleep training around age 3 to 4 because my need for 'me time' began to outweigh my need to help them feel safe and protected and loved. So i fully understand why most others in the Western world, do it differently.
I just wanted to contextualise where your son is coming from and this might help you to 'step into his shoes' for a bit and respond differently. Good luck