My DD had a lot of anxiety around bedtime. We did a course through our local mental health NHS team around parenting children with anxiety as it was impossible to deal with ourselves.
Essentially what we did was plan our end goal and then work backwards in steps to plan how to get there. Each week we would try to advance a step. This was accompanied with teaching DD techniques like breathing exercises, grounding techniques like look for 5 things you can touch, 4 you can see, 3 you can hear, 2 smell etc. less successful was trying to "challenge" her worries eg why does that worry you/ what would happen next etc. We were told to be clear with DD on what our goal was and the next step we would next do and when.
The hardest bit was me trying to not feed her anxiety by anticipating it as this didn't feel like my natural parenting style to warn her "you might find this hard" and comfort her
The overall approach of going up the exposure ladder did work . We still do the same routine now, eg read, I stay for 5 mins to get any "worries "out of her head and then we go. She had print outs on her wall reminding her of the coping mechanisms temporarily but she doesn't need them much now.
Having said all if this, there is no shame in getting the double bed for his room, embracing that he wants to be close to you at night and waiting for him to grow out of it. It's whatever suits your family best. A friend's daughter slept in her parents room until she was 12 but happily is in her room now and did it of her own accord.