Hi there. After dealing with RSV with my premie twin boys and nearly losing one at 8 weeks to the virus, I spent about 6 years being a parent advocate for RSV awareness. It is still amazing to me that the entire world knows about SIDS but for some reason the medical community doesn't make as much noise about RSV.
Take a deep breath because things will be ok. EVERY CHILD WILL ENCOUNTER RSV BEFORE THE AGE OF TWO. The issues come in to play with how each child's body reacts to the virus. Most times premies are not as able to fight off the virus as a full term baby. The fact that your son is six months old, not struggling to breathe and that he is not lethargic are great indicators that he is equipt to fight this virus. I am assuming that he is on some antibiotic although your post didn't indicate that. Continue with the meds but if you do not see any significant improvement by the end of the course, request another round of something stronger. You are his mother and you know him and his mannerisms better than anyone else...doctor or not.
Make sure that everyone coming into contact with him practices good handwashing proceedures, especially your 4 year old. If she goes to pre-school, make sure to wash her hands really well as soon as she comes home. Also, due to the bronchiolitis make sure he is not around anyone smoking or being held up against clothing of anyone who smokes.
Hopefully you will not have any future issues from this episode, but be forewarned that asthma and repeat bronchitis can be results of having had RSV. My boys are 11 and are still dealing with the asthmatic results of RSV.
If you would like to contact me via email, feel free to do so at ____@____.com will both get through this with flying colors!