S.B.
Kids who don't know how to blow their nose tend to vomit the phlegm to get rid of it. It's a common response, especially if your child has a strong gag reflex. Our now four year old had this happen to him a lot last winter. He was three at the time and every night he would vomit as the mucus stayed in his throat and would irritate him. (Think about how uncomfortable you feel when you have mucus in your thoat- you know how to spit it out, they don't).
For us, it progressed from nightly vomiting, to twice, then three times a day, for a peroid of two months (as long as the cold lasted). So, in other words, for us it got a lot worse. Because he was vomiting the phlegm up so much, his esophagus got more and more irritated, which meant more stuff would bother him, he would vomit more, and finally ended up with something more like acid reflux. He had to take medicine to heal the esophagus. And all this because the mucus caused a gag reflex..... It was really scary for us, he lost a lot of weight; and started becoming a picky eater. But now that he knows how to blow his nose right, he no longer has this problem. Moral of the story is, the doctors will tell you it's normal, but it can become a catch 22 (vomit to get rid of mucus, irritate esophagus, keep vomiting). Hopefully your son's cold won't last as long as my son's did, and he'll learn how to blow his nose sooner. :)
Good luck.
A little about me:
I have three boys. A 6 year old, a 4 year old, and a 6 month old I am totally inlove with (we were trying for a girl too).