Hi there, I've never dealt with RSV but my 8 year-old has asthma, which is basically lifelong RSV. She started having severe flares that required hospital intervention at your son's age now, so I can pass along a few tips that may help.
When he's coughing, coughing, coughing and won't stop, try sitting in the bathroom with him while the shower's running and the door's closed, just like parents used to do (some still do) for croup. The hot steam helps to loosen up all the snot--just don't stay in there too long, only 10 min. or so.
Another thing that sometimes helps during croup and/or asthma is to expose a child to a drastic change of temperature. I have NO idea why this works (and I realize how odd it sounds) but it does sometimes. When my daughter couldn't stop coughing during winter and was breathing too fast AND we had used all the breathing treatments we could but she wasn't quite bad enough to warrant an ER trip, we'd bundle her up and let her breathe the cold air outside for a couple of minutes. For whatever reason, the cold air seems to help shock the system into calming down or something. The asthma parents who told me this on another board years ago would even resort to standing in front of an open refrigerator during the summer for particularly bad flares.
Also, ask your pediatrician about doing chest percussion therapy. We learned how to do it from a respiratory therapist in the hospital, but I'm unsure of the procedure for babies. I know some drs. don't necessarily think it helps, but it's worth asking about.
Hope that helps, and I hope he starts feeling better soon! I know how scary this stuff can be with a baby.
