E.A.
We didn't transition our kids to their own rooms until they were 3yo. I don't know why you think you made a mistake in letting him sleep with you, I certainly don't think you did. It's just a different way of parenting. I say let him stay in your room in his bed for a little while longer, and then move the bed to his room, lie down with him to go to sleep, and let him come in with you when he wakens in the middle of the night. After a while of this, say a few months, start bringing him back to bed when he comes in with you. You can start to room your kids together around his time, and they will find comfort in that. You can even let them sleep together, my kids did that when they were little. Only one of my children had a hard time after 3yo sleeping alone, but with positive reinforcement and lots of love, she has recently learned to stay in her bed all night. I probably could have worked with her sooner to accomplish this, but it wasn't until the last year that anyone really minded her habit of coming in with us in the middle of the night. And anyway, they grow up so fast, they really do, and I treasure those memories of waking up to a sleeping baby/toddler.