I was actually going to suggest a baby gate up at the door. My LO was younger, but it was a necessity for us, he simply could not stay in his room. After a couple of days of battle, he settled quickly and easily, and still (a year later) asks for his gate to be shut if I leave it open. I think it helps him to feel more secure.
Five minutes is a LONG time for a little guy, especially if he's feeling tired and/or anxious. I would say you will be back in 'a minute' and go back in 30 seconds. Then 40. Then 50. Then a minute. Then two minutes. Keep going back every two minutes.
Other thoughts:
Talk about bedtime/nap time when you are not 'in the moment'. Let him know what your expectations are, and WHY you need him to go to sleep ('I need to help you to get the rest that your body needs, so we can both be happy')
A pictorial chart showing his wind-down routine, ending up with him sleeping happily in bed, so he feels that he has some control
Making sure his 'mummy cup' is REALLY full. I have heard that a lot of anxiety about a younger sibling really starts to kick in as they get older. When they are tiny they don't really 'do' much, but the more cute and engaging the little one gets, the more the older one sees his mum and dad giggling and cooing and trying to get smiles from the little one
A longer wind down. Once we got to 2.5, I really had to extend the wind-down. More stories, taking longer. I don't know if that's even feasible with a younger one, but just throwing it out there.