Is there a soft approach we can take?
Yes. It's called gradual withdrawal.
What do you do at the minute? Say you sit by his bed and it take a 30 minutes. So tomorrow deliberately take the bath towel (or whatever) into his room after the bath. Do bedtime as normal, settle him for 10 minutes then say you're just popping the towel back, be straight back. Only be gone 30 seconds then stay as normal.
He'll probably not trust you'll come back at first, hence why you need to be super quick. Then don't leave again, stay until asleep. Do that for several days, each day finding an excuse to pop out just for half a minute or so.
The aim is for him not to get worried or upset when you say you are popping out the room, because he trusts you to come back. Only when you have this trust do you move on.
I'd be inclined to develop a little job you do every night upstairs. put washing away, clean bathroom, gold your clothes. A predictable "job" that you always pop out to do but then come back.
Then over time you stay out the room longer. Maybe a minute at first. Then go back to him and stay until asleep. Then 2 minutes.
Once he's used to you going out the room, develop excuses for multiple trips out. So "I'm just folding my clothes, back in a minute". Come back and whisper, I didn't finish folding the clothes, is it ok if I just go back and finish them again, I promise I'll come back", kind of thing.
Then settle him, tell him you are going to do whatever upstairs, but keep popping your head around the door to say you are still her and doing the job (whatever it is)
Make it so you maybe spend a couple of minutes settling him, then go do a job upstairs but pop your head in the doorway every couple of minutes so he knows you are there, going back into his room properly only if awake still after 30 minutes.
The key really is trust. He needs to know that if he needs you, you'll be right there in his room for as long as he needs you. But that trust is 2-way and he also needs to know that when he is ok and doesn't need you, that you can be nearby but don't have to be in his room. That he can always call you if needed.