N.,
My daughter is a preemie (born 2 mos early) and she's almost 22 months old. From a developmental standpoint, she has been about 2-4 months late with a lot of things. Not everything, some of the more complex things she's done early, but a lot of the motor-skill things. With rolling over, she was almost 7 months old before she really got the hang of it. She just started consistently walking everywhere about three weeks ago, right when I was about to pull my hair out (just kidding about the hair).
When Dr.'s look at skills that babies ought to be hitting, they ask whether the kiddo can do it, and if not, is the kiddo showing "emerging skills", like signs that they are on their way to doing it. Like would your son sort of twist over sideways to grab a toy, if so, that's an emerging skill. My Dr. has been kind of relaxed about most of our milestones because of the fact that there has always been evidence of "emerging skills" and of course we were factoring in her adjusted age due to prematurity. So we set boundaries, like, yes, baby should roll over around 6 months but if she isn't doing it by 8 months, then we are going to set up ECI. And she always has come in just under the wire with whatever skill we were watching for.
I don't think holding your son really affects his ability to roll over, but I do think it would be good to spend a little time with him over the next few days on the floor. Lie him down on his back and hold attention-grabbing toys up over him to try to get him to reach for them, then slowly bring it down to each side, trying to get him to reach for them and twist over. Don't spend more than 5 or 10 minutes at a time doing this. That is what we did to help our daughter on this one.
As for ECI, I don't have any experiences personally, but everyone I know that's had them have had nothing but great things to say about it.
I do find it sort of strange that your son is on the mark with all his other milestones and the Dr. is worried about this so soon. Every kid is different and some things they do early, some late, some on time. Work with your son, be sure to ask the Dr. plenty of questions including recommendations for excercises or games you can play with him to help him on this, and don't sweat the ECI. If your son's on track, likely that will be a very temporary situation anyway as he will probably pick this up super-quickly with their help if he hasn't figured it out before then.
Good luck, let us know what happened!