Why don't you just put the railing back on? If it makes him feel secure, that's the first thing I would do. If you think he's insecure without it, then yes, the simplest option is to put it back on. If that is the crux of the problem, it's a very easy fix. For what it's worth, my son used a side rail until he was well past four. There's no magic time that he needs to get rid of it... just when he's ready.
If you ever want to try it~
The "just put them back to bed" approach is not a one-night, one time option. You have to decide that you are willing to spend a week or more if need be, teaching him that once he's in bed that he must stay in bed and not pop out for every little thing. This would mean maybe a sippy cup with a valve (to stop leaks) on a night table. Otherwise, I did it much like I did boot came, with great resolve and no looking back. I made the commitment to teach my son what he needed to learn and then actually camped out in the hallway, so that every time he got out of bed, I quietly walked him back to bed. No talking, no cuddles, no eye contact, no angry expressions on my face. NOTHING. The first time, I just said 'bedtime' and took him back to bed. Then I did that about four more times before he realized that he wasn't going to wear me out.
Tricks:
DVR anything you want to watch that night. Be devoted to your cause. You are teaching your son that he is fine sleeping on his own.
Make it pleasant for yourself. I actually put down my yoga mat and some blankets and pillows in the hallway and had books to read, my laptop and my crossword puzzles. I didn't feel like I was missing out on the evening.
Stay calm and matter of fact about this. "Bed" in a matter-of-fact voice and that's it. Keep putting him back to bed. No talking. No chatting, no threats or punishments.
(I should add that re-reading your post, I now catch the broken ankle part. Maybe your husband should be doing this 'teaching' for now and you stay in another part of the house out of sight.)
DO try replacing the side rail first, though. It seems like this could be an easy fix.