L.L.
Vomiting mucus (or having it come out the other end) CAN be a normal event...like someone mentioned earlier, babies can't blow their nose or "hack it up" in their throat properly, so it gets swallowed.
However, something may be causing excess mucus that needs to be evaluated. If he isn't feeling better and done throwing up by the morning, I would take him in. Hope he feels better soon!