Is it shortly after he has eaten? Maybe he didn't get enough of a burp out before you laid him down and a little gas just came out really forcefully.
My first dd had slight reflux, and we had to hold her upright for about 10-15 minutes after she nursed so she wouldn't cough and spit up as much. I remember each of my girls having spit up come out their nose at least once, and I remember being worried at the time, but I don't think it's too big of a deal. If it really worries you though, don't be afraid to call your pedi or the "ask a nurse" line if your hospital has one. They can help put your mind at ease.
at 11:00 PM on Jan. 19, 2010