Does he play in his room while you are around (in and out of his room folding laundry or cleaning or whatever else you do nearby)?
Get him to see his room as a fun place to be, perhaps spend time in there playing toys or reading books sitting or lying on his bed
So gradually each day spend more time in there doing puzzles or reading books or playing with toys than you do in other rooms
Not putting any pressure on him
Put your baby to sleep in his/her own cot in the other bedroom so ds1 doesn't miss out on being in your room/bed while baby is still in your room
You could let him have your duvet cover on his bed so your smell is in there
Or you could go down the route of you sleeping in his bedroom on mattress and gradually moving it towards the door
Or just go cold turkey and put him back in his own bed each night but that will be hard work for you and emotional and could take a few weeks but it might work quickest
It will be hard for him especially with the baby getting your attention but you could end up with both of them in your bed in 6 months time if you delay
Get him to choose some new bedding with Thomas or animals or whatever design is his favourite or some posters with the characters or animals on
So he gets excited about his room being his iyswim