That's more useful information to be able to offer suggestions to help, I wasn't being needlessly nosy with all my questions
OK, this is primarily a behaviour issue rather than a sleep development issue. You deal with it like you would any other unwelcome behaviour- strict boundaries, no negotiation, clear expectations. You effectively need to get strict with him.
Expectations
Your expectations is not that he goes to sleep - that's not something you can control.
Your expectation should be: you lie down in bed at sleep time. This is what you need to be repeatedly communicating to him.
The idea is that once be accepts that nothing else can happen but lying in bed, boredom will get the better of him and he will nod off (hence don't provide toys in bed, just comforters)
Boundaries
This is you taking no shit and being strict. I would stop being in the room with him. He doesn't need you there and you're just a distraction.
But he does need to know you're close by. Both for reassurance and also there to keep strict boundaries and the rules that go with that. So it have his bedroom door open and you in the hallway. You need to position yourself so you can either see or hear any movement, any time he gets up.
Then it is a case of being repetitive and consistent.
Lie him down. Leave. Lie him down. Leave. Lie him down. Leave. Lie him down. Leave. Over and over and over again.
He may well be a proper git about it at first. But stay calm, don't get cross. Just keep to your boundaries and expectations- he must lie down in bed at sleep time. End of, nothing else is acceptable.
No Negotiation
These are all the distraction techniques he might use. I need the toilet, I want a cuddle, I need a drink. There are a gazillion things toddler a come up with.
Try to preempt as much as possible. Ensure he's had a drink, had a wee or clean nappy, got everything he needs. Plenty of cuddles before bedtime so you can't be guilt tripped into thinking you don't cuddle him enough.
Then just say no. Don't get into any form of discussion or negotiation. Just the expectations - you will lie down in bed (on his own). That's the single only thing that matters.