My youngest (who is now 1) had it really, really bad last year. He was full-term and 6 weeks when he got it but was in the NICU at CHOP for a week. This is the basics to know:
It is a virus and it usually peaks about days 5-7. Sometimes it can peak as late as 8-10 days (from the first onset of symptoms) and can linger for awhile.
The thing that caught my attention is that he is having trouble breathing. My son never ran a fever and did need oxygen. We had him back in the hospital 2x during it because he got so bad he was sleeping all the time and pretty lethargic.
IF YOU HAVE ANY DOUBTS, TAKE HIM BACK TO THE ER OR DOCTOR. You don't want to mess with this because it can be nothing, but also can be pretty seriously.
Watch his stomach:
if he is pulling in when he breathes, (you can see the outlines of his individual ribs)
if he seems to be working really hard to breath
if he is sucking in more than normal when he breathes at his neck
or if you are just concerned, it is worth at least taking him back to the doctor. The offices can check his oxygen levels usually and tell you how he is doing.
I don't want to scare you, but this can be really serious. If you feel things are not right, make them check him out again. You pay them to do that and that is there job.
We got sent home the first visit (we were out of town at a hospital by our friend's house) and the doctors there made us feel really stupid - like we were worried for nothing. Two days later I knew he was getting bad and I am glad I trusted my instinct, because when we took him into the other ER, he was pretty bad and the docs said he really needed to be there.
That being said, the biggest thing they can do for him there is give him oxygen if he needs it and other supporting therapies to help him get over this. Viruses do not respond to other forms of treatments so they will not give you antibiotics unless they suspect an infection.
Best of luck and feel free to write with any questions on our experience. I hope he gets over it soon!