We had to put DS in a proper bed last year when we weny on hols and he was too big for the cot. He absolutely loved his 'big boy' bed and we got him his own as soon as we came home. Agree with the stairgate on the door. You can get really lightweight toddler duvets he might like - with a colourful cover on. You could try putting him on top of duvet at first with a blanket over the top then popping him under it when he's sound asleep - he might get used to the feel of it over him better
Oh perfect timing! My DS1 moved into a toddler bed when I was pregnant too. He could see the reasoning behind it, as I said the baby needs the cot soon. So we moved it from his room but still had it in the house. He liked the fact that he was big enough to get a proper bed. Took him to Ikea and he chose the duvet cover and bed. I put a stairgate on his door, and after 2 nights of confusion, worked a treat. Still had occasional late evenings of getting him back into bed, but it goes soon.
We've set up a new room from him with all his favourite toys in it as the baby room is too small. He's been using it as a playroom for the past fortnight until his bed arrives and he loves it. Makes a beeline for it as soon as he's out of his cot. He's only 20 months.
DH has convinced him that the light is magic though as he can't see the switch and stands in the middle of the room saying light on while DH operates the switch so I guess we'll get tears of frustration at some point soon.
I'll take anothe look at duvets in a month or so having just spent loads on a bed and bed linen - I'm skint!